Panic when doing 'ipfs cat'

[chriscool]

This comes from this Circle CI run:

https://circleci.com/gh/ipfs/go-ipfs/2632

for PR #2208.

The error is:

expecting success: 
        ipfs cat "$PCOUT/$name" >tpcp_out &&
        ipfs cat "$target" >tpcp_exp &&
        test_cmp tpcp_exp tpcp_out

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x40 pc=0x5b4fbe]

goroutine 1 [running]:
github.com/ipfs/go-ipfs/repo/fsrepo.(*FSRepo).GetStorageUsage.func1(0xc8200bc7e0, 0x5d, 0x0, 0x0, 0x2b019bdc4270, 0xc82068c0c0, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/repo/fsrepo/fsrepo.go:565 +0x3e
path/filepath.walk(0xc8200bc1e0, 0x52, 0x2b019c202890, 0xc8200b42d0, 0xc8201797c0, 0x0, 0x0)
    /usr/local/go/src/path/filepath/path.go:370 +0x415
path/filepath.walk(0xc8200101ea, 0x48, 0x2b019c202890, 0xc820016af0, 0xc8201797c0, 0x0, 0x0)
    /usr/local/go/src/path/filepath/path.go:374 +0x4fc
path/filepath.Walk(0xc8200101ea, 0x48, 0xc8201797c0, 0x0, 0x0)
    /usr/local/go/src/path/filepath/path.go:396 +0xe1
github.com/ipfs/go-ipfs/repo/fsrepo.(*FSRepo).GetStorageUsage(0xc820018940, 0x2, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/repo/fsrepo/fsrepo.go:567 +0xa7
github.com/ipfs/go-ipfs/repo.(*ref).GetStorageUsage(0xc8200e4780, 0x411358, 0x0, 0x0)
    <autogenerated>:22 +0x5c
github.com/ipfs/go-ipfs/core/corerepo.(*GC).maybeGC(0xc8206e4580, 0x2b019bdc4998, 0xc820018900, 0xc, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/core/corerepo/gc.go:162 +0x65
github.com/ipfs/go-ipfs/core/corerepo.ConditionalGC(0x2b019bdc4998, 0xc820018900, 0xc820116000, 0xc, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/core/corerepo/gc.go:158 +0x86
github.com/ipfs/go-ipfs/core/commands.glob.func18(0x2b019bdc48c0, 0xc820298240, 0x2b019bdc4a28, 0xc820014150)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/core/commands/cat.go:48 +0x2a8
github.com/ipfs/go-ipfs/commands.(*Command).Call(0x1429e40, 0x2b019bdc48c0, 0xc820298240, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/commands/command.go:110 +0x471
main.callCommand(0x2b019bdc4998, 0xc820018840, 0x2b019bdc48c0, 0xc820298240, 0x1429e40, 0x1418e80, 0x0, 0x0, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/cmd/ipfs/main.go:343 +0xa5c
main.(*cmdInvocation).Run(0xc820018800, 0x2b019bdc4998, 0xc820018840, 0x0, 0x0, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/cmd/ipfs/main.go:186 +0x1e5
main.main()
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/cmd/ipfs/main.go:151 +0x65b

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64.s:1696 +0x1

goroutine 18 [chan receive]:
github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/codahale/metrics.init.1.func2()
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/codahale/metrics/metrics.go:321 +0x73
created by github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/codahale/metrics.init.1
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/codahale/metrics/metrics.go:328 +0x7b

goroutine 19 [syscall]:
os/signal.loop()
    /usr/local/go/src/os/signal/signal_unix.go:22 +0x18
created by os/signal.init.1
    /usr/local/go/src/os/signal/signal_unix.go:28 +0x37

goroutine 20 [select]:
github.com/ipfs/go-ipfs/vendor/QmQg1J6vikuXF9oDvm4wpdeAUvvkVEKW1EYDw9HhTMnP2b/go-log.(*MirrorWriter).logRoutine(0xc8200ee580)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/vendor/QmQg1J6vikuXF9oDvm4wpdeAUvvkVEKW1EYDw9HhTMnP2b/go-log/writer.go:71 +0x33c
created by github.com/ipfs/go-ipfs/vendor/QmQg1J6vikuXF9oDvm4wpdeAUvvkVEKW1EYDw9HhTMnP2b/go-log.NewMirrorWriter
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/vendor/QmQg1J6vikuXF9oDvm4wpdeAUvvkVEKW1EYDw9HhTMnP2b/go-log/writer.go:38 +0xe2

goroutine 8 [select, locked to thread]:
runtime.gopark(0x1088c00, 0xc82002c728, 0xecb298, 0x6, 0x43cd18, 0x2)
    /usr/local/go/src/runtime/proc.go:185 +0x163
runtime.selectgoImpl(0xc82002c728, 0x0, 0x18)
    /usr/local/go/src/runtime/select.go:392 +0xa64
runtime.selectgo(0xc82002c728)
    /usr/local/go/src/runtime/select.go:212 +0x12
runtime.ensureSigM.func1()
    /usr/local/go/src/runtime/signal1_unix.go:227 +0x353
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64.s:1696 +0x1

goroutine 29 [runnable]:
github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/jbenet/goprocess.(*process).doClose.func1(0x0, 0x0, 0x0)
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/jbenet/goprocess/impl-mutex.go:208
created by github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/jbenet/goprocess.(*process).doClose
    /home/ubuntu/.go_project/src/github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/jbenet/goprocess/impl-mutex.go:221 +0x309
not ok 50 - output looks good

[whyrusleeping]

This has been fixed, it was a bug in the walk function for the datastore enumeration.