compiler: Don't crash on erroneous array return types.

compiler: Don't crash on erroneous array return types.

    Another issue with erroneous array types.  When an erroneous array
    type is in a function's signature, particularly the return type, we
    must guarantee that type is changed into an error type.  Otherwise,
    any operations that work on arrays and slices will crash when applied
    to the erroneous array return type.

    Fixes golang/go#12939.

    Reviewed-on: https://go-review.googlesource.com/16235

From-SVN: r232858