libhandle: fix potential unterminated string problem

libhandle: fix potential unterminated string problem

gcc 10.2 complains about the strncpy call here, since it's possible that
the source string is so long that the fspath inside the fdhash structure
will end up without a null terminator.  Work around strncpy braindamage
yet again by forcing the string to be terminated properly.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>