os/user: on AIX getpwuid_r seems to return -1 on overflow
    
    The getpwuid_r function is expected to return ERANGE on overflow.
    Accept -1 on AIX as we see that in practice.
    
    This problem was uncovered by, but not caused by, CL 455815,
    which introduced a test that forced a buffer overflow.
    
    Change-Id: I3ae94faf1257d2c73299b1478e49769bb807fc4d
    Reviewed-on: https://go-review.googlesource.com/c/go/+/456075
    Auto-Submit: Ian Lance Taylor <iant@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@google.com>
    Reviewed-by: Bryan Mills <bcmills@google.com>
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gopher Robot <gobot@golang.org>