don't pass null to switch_loadable_module_get_api_interface. Found by Klockwork (www.klocwork.com)

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@8439 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/switch_loadable_module.c

@@ -1349,7 +1349,7 @@ SWITCH_DECLARE(switch_status_t) switch_api_execute(const char *cmd, const char *
 		}
 	}
 
-	if ((api = switch_loadable_module_get_api_interface(cmd)) != 0) {
+	if (cmd && (api = switch_loadable_module_get_api_interface(cmd)) != 0) {
 		status = api->function(arg, session, stream);
 	} else {
 		status = SWITCH_STATUS_FALSE;