libmount: more robust options string parsing

 # mount -o=rw /dev/sdb /mnt/test
 mount: libmount/src/optmap.c:212: mnt_optmap_get_entry: Assertion `namelen' failed.
 Aborted (core dumped)

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=968786
Signed-off-by: Karel Zak <kzak@redhat.com>