gccgo: internal compiler error in check_bounds

[dvyukov]

gccgo crashes on the following input:

package a
var g = "0"[0i]

go1: internal compiler error: in check_bounds, at go/gofrontend/expressions.cc:486
0x5e5b99 Expression::check_bounds(Expression*, Location)
    ../../gcc/go/gofrontend/expressions.cc:486
0x5e6def String_index_expression::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/expressions.cc:10352
0x5f0d56 Type_conversion_expression::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/expressions.cc:3256
0x61db91 Variable::get_init_block(Gogo*, Named_object*, Bvariable*)
    ../../gcc/go/gofrontend/gogo.cc:6392
0x626337 Gogo::write_globals()
    ../../gcc/go/gofrontend/gogo.cc:1418

gcc version 6.0.0 2015070 (experimental) (GCC)

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/13796 mentions this issue.