fix runtime in older versions of PHP caused by json_last_error_msg

fulcrm_webhook.module

@@ -151,7 +151,10 @@ function fulcrm_webhook_webhook( $uuid ) {
         if ( array_key_exists( 'Content-Type', $headers ) && ( $headers[ 'Content-Type' ] === 'application/json' ) ) {
             $payload = json_decode( file_get_contents( "php://input" ), $assoc = TRUE );
             $payload_error = json_last_error();
+            if ( function_exists( 'json_last_error_msg' ) )
                 $payload_error_msg = json_last_error_msg();
+            else
+                $payload_error_msg = 'JSON error unavailable';
         } else {
             $payload_error_msg = 'no payload';
         }