PR go/90110
    compiler: use temporary to avoid early destruction
    
    The code was passing a substr directly to strtol, and then checking
    the *end value returned by strtol.  But the substr could be destroyed
    as soon as strtol returns, making the test of *end invalid.
    
    Also fix an incorrect test of the string index rather than the value.
    
    Fixes https://gcc.gnu.org/PR90110
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/172663

From-SVN: r270434