Error out on negative indices in element()

[billf]

prevents a negative offset in StringList.Element():

return sl.Slice()[index%sl.Length()]

fixes the following crash:

panic: runtime error: index out of range

goroutine 5424 [running]:
panic(0xa00f20, 0xc82001a160)
    /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:464 +0x3e6
github.com/hashicorp/terraform/config.StringList.Element(0xc8209c6300, 0x16a, 0xfffffffffffffffb, 0x0, 0x0)
    /Users/bill/goprojects/src/github.com/hashicorp/terraform/config/string_list.go:60 +0xef
github.com/hashicorp/terraform/config.interpolationFuncElement.func1(0xc820654e40, 0x2, 0x2, 0x0, 0x0, 0x0, 0x0)
    /Users/bill/goprojects/src/github.com/hashicorp/terraform/config/interpolate_funcs.go:483 +0x2bd
github.com/hashicorp/terraform/vendor/github.com/hashicorp/hil.(*evalCall).Eval(0xc820ab63a0, 0x24c0a48, 0xc820e8d290, 0xc820f14dd0, 0x0, 0x0, 0x139aa58, 0x0, 0x0)
    /Users/bill/goprojects/src/github.com/hashicorp/terraform/vendor/github.com/hashicorp/hil/eval.go:179 +0x41f
...

[jen20]

Hi @billf! Thanks for opening this pull request - good catch!

[ghost]

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