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!!)
    
    (cherry picked from commit fba019481012b18818d07eb729469d29cb1a5e5d)