Ticket #48279 - Check NULL reference in nssasl_mutex_lock etc. (saslbind.c)
Description: Some SASL subsystem are thread safe and do not require
locking. This may call callbacks in saslbind.c with NULL mutex.
This patch adds NULL checking before calling PR_.*[Ll]ock.
https://fedorahosted.org/389/ticket/48279
Reviewed by mreynolds@redhat.com (Thank you, Mark!!)