gccgo: segfault

[h4ck3rm1k3]

This does not crash on my debian amd64, but on the ppc64le arch with the latest gcc.
The code is from the core go lib. src/runtime/print1.go
Part of the #10092 task.

@ianlancetaylor here is the reduced bug report.

package main
func gwrite(b []byte) {
        n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
}

produces the stacktrace
gccgo (GCC) 5.0.0 20150328 (experimental)(103cf1a425ceb970ef376580800ec19845a378f0)
Linux gcc2-power8.osuosl.org 3.17.4-301.fc21.ppc64le #1 SMP Mon Dec 1 07:51:01 UTC 2014 ppc64le ppc64le ppc64le GNU/Linux

 ~/install/bin/gccgo testcase.go 
testcase.go:5:12: error: reference to undefined name ‘gp’
  n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
            ^
testcase.go:5:28: error: reference to undefined name ‘gp’
  n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
                            ^
testcase.go:5:45: error: reference to undefined name ‘gp’
  n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
                                             ^
testcase.go:5:2: error: ‘n’ declared and not used
  n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
  ^
go1: internal compiler error: Segmentation fault
0x107f0433 crash_signal
        ../../gcc/toplev.c:383
0x1026bf80 Array_type::get_value_pointer(Gogo*, Expression*) const
        ../../gcc/go/gofrontend/types.cc:6163
0x101d4ddf Builtin_call_expression::do_get_backend(Translate_context*)
        ../../gcc/go/gofrontend/expressions.cc:8353
0x101c2c47 Expression::get_backend(Translate_context*)
        ../../gcc/go/gofrontend/expressions.cc:411
0x101d846b Type_conversion_expression::do_get_backend(Translate_context*)
        ../../gcc/go/gofrontend/expressions.cc:3340
0x101c2c47 Expression::get_backend(Translate_context*)
        ../../gcc/go/gofrontend/expressions.cc:411
0x1020aebb Variable::get_init(Gogo*, Named_object*)
        ../../gcc/go/gofrontend/gogo.cc:6195
0x1025ec8f Variable_declaration_statement::do_get_backend(Translate_context*)
        ../../gcc/go/gofrontend/statements.cc:267
0x10253797 Statement::get_backend(Translate_context*)
        ../../gcc/go/gofrontend/statements.cc:141
0x10216e4f Block::get_backend(Translate_context*)
        ../../gcc/go/gofrontend/gogo.cc:5648
0x10217bd7 Function::build(Gogo*, Named_object*)
        ../../gcc/go/gofrontend/gogo.cc:5254
0x10218503 Named_object::get_backend(Gogo*, std::vector<Bexpression*, std::allocator<Bexpression*> >&, std::vector<Btype*, std::allocator<Btype*> >&, std::vector\
<Bfunction*, std::allocator<Bfunction*> >&)
        ../../gcc/go/gofrontend/gogo.cc:6973
0x1022058f Gogo::write_globals()
        ../../gcc/go/gofrontend/gogo.cc:1289
0x102045bf go_write_globals()
        ../../gcc/go/gofrontend/go.cc:147
0x101fe727 go_langhook_write_globals
        ../../gcc/go/go-lang.c:463
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.

[ianlancetaylor]

Fixed on tip.

Note that this is a crash on invalid code, and that is what I fixed. I don't know if there is another compiler crash on valid code somewhere.