[krbdev.mit.edu #3490] getpwnam_r status checked incorrectly

[Public Submitter via RT]

glibc's getpwnam_r returns success even if the user wasn't found, but
the result pointer is set to NULL.  The Kerberos source assumes that
k5_getpwnam_r will fail if the user wasn't found and dereferences OUT
without any further checks.  Use a technique similar to the other cases
and change the status to -1 if OUT is NULL.

A better approach may be to change the source to not assume success
means that OUT is non-NULL, since that appears to be all POSIX
guarantees.  But this works.