compiler: avoid compiler crash on invalid program

I encountered this crash while working on upgrading libgo to the 1.9
release.  I no longer have the cause of the crash, but it doesn't much
matter, as the policy for crash-on-invalid errors is to fix the crash
but not bother to commit the invalid test case.

Change-Id: I0b971dad0865114435b2bb43f8b8899d67b47e33
Reviewed-on: https://go-review.googlesource.com/62750
Reviewed-by: Than McIntosh <thanm@google.com>