compiler: check error expression in Array_type::get_backend_length
compiler: check error expression in Array_type::get_backend_length

Otherwise, a zero length is created in the backend and the
backend doesn't know there is an error.

Change-Id: Ifd285963e0cab8498f631cfaf70548027a9c6310
Reviewed-on: https://go-review.googlesource.com/61610
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend