rtld: Fix null-pointer dereference

When a library is opened via fdlopen, it has a null pointer for its path
and so _rtld_bind can crash as a result of passing the null pointer to
basename() (which passes it to strrchr(), which doesn't do a null check).

PR: 253081
Submitted by: theraven
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28442