lock.c: Check *lt before dereferencing it

*lt is NULL if t->tracking == 0

ASTERISK-25948 #close

Change-Id: I4a81af28f9c82a74aa82413d772a7dc8fa6f45ba

main/lock.c

@@ -286,6 +286,7 @@ int __ast_pthread_mutex_lock(const char *filename, int lineno, const char *func,
 				if (wait_time > reported_wait && (wait_time % 5) == 0) {
 					__ast_mutex_logger("%s line %d (%s): Deadlock? waited %d sec for mutex '%s'?\n",
 							   filename, lineno, func, (int) wait_time, mutex_name);
+					if (lt) {
 						ast_reentrancy_lock(lt);
 #ifdef HAVE_BKTR
 						__dump_backtrace(&lt->backtrace[lt->reentrancy], canlog);
@@ -297,6 +298,7 @@ int __ast_pthread_mutex_lock(const char *filename, int lineno, const char *func,
 						__dump_backtrace(&lt->backtrace[ROFFSET], canlog);
 #endif
 						ast_reentrancy_unlock(lt);
+					}
 					reported_wait = wait_time;
 				}
 				usleep(200);