res_xmpp: fix inverted return code check in OAuth

fetch_access_token calls func_curl via ast_func_read. The latter returns 0 upon
success and -1 if the function is not available.
This commit inverts the return code check so that an error is printed if the
module is not loaded and not if it is loaded.

ASTERISK-27207 #close

Change-Id: I9ef903f80702d1218e8701f65a4e5e918e6548fb

res/res_xmpp.c

@@ -3917,7 +3917,7 @@ static int fetch_access_token(struct ast_xmpp_client_config *cfg)
 	ast_debug(2, "Performing OAuth 2.0 authentication for client '%s' using command: %s\n",
 		cfg->name, cmd);
 
-	if (!ast_func_read(NULL, cmd, cBuf, sizeof(cBuf) - 1)) {
+	if (ast_func_read(NULL, cmd, cBuf, sizeof(cBuf) - 1)) {
 		ast_log(LOG_ERROR, "CURL is unavailable. This is required for OAuth 2.0 authentication of XMPP client '%s'. Please ensure it is loaded.\n",
 			cfg->name);
 		return -1;