compiler: avoid crash on erroneous type

compiler: avoid crash on erroneous type

    If there is an error constructing the backend type, the GCC backend
    will report that the size is 1.  That will then cause construction of
    the ptrmask to crash.  Avoid that case by just generating an empty
    ptrmask.

    Noticed while compiling a broken package.  The policy I've been
    following is to not commit a test case for a compiler crash on invalid
    code, so no test case.

    Reviewed-on: https://go-review.googlesource.com/45775

From-SVN: r249208