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.
    
    Reviewed-on: https://go-review.googlesource.com/62750

From-SVN: r252748