res_corosync: Add check for config file before calling corosync apis

On some systems, res_corosync isn't compatible with the installed version of
corosync so corosync_cfg_initialize fails, load_module returns LOAD_FAILURE,
and Asterisk terminates.  The work around has been to remember to add
res_corosync as a noload in modules.conf.  A better solution though is to have
res_corosync check for its config file before attempting to call corosync apis
and return LOAD_DECLINE if there's no config file.  This lets Asterisk loading
continue.

If you have a res_corosync.conf file and res_corosync fails, you get the same
behavior as today and the fatal error tells you something is wrong with the
install.

ASTERISK-24998

Change-Id: Iaf94a9431a4922ec4ec994003f02135acfdd3889

res/res_corosync.c

@@ -883,6 +883,12 @@ static int load_module(void)
 		goto failed;
 	}
 
+	if (load_config(0)) {
+		/* simply not configured is not a fatal error */
+		res = AST_MODULE_LOAD_DECLINE;
+		goto failed;
+	}
+
 	if ((cs_err = corosync_cfg_initialize(&cfg_handle, &cfg_callbacks)) != CS_OK) {
 		ast_log(LOG_ERROR, "Failed to initialize cfg: (%d)\n", (int) cs_err);
 		goto failed;
@@ -913,12 +919,6 @@ static int load_module(void)
 		goto failed;
 	}
 
-	if (load_config(0)) {
-		/* simply not configured is not a fatal error */
-		res = AST_MODULE_LOAD_DECLINE;
-		goto failed;
-	}
-
 	ast_cli_register_multiple(corosync_cli, ARRAY_LEN(corosync_cli));
 
 	return AST_MODULE_LOAD_SUCCESS;