gs-lock-plug.c: Fix build warning:

ignoring return value of ‘fscanf’, declared with attribute warn_unused_result

src/gs-lock-plug.c

@@ -190,7 +190,10 @@ process_is_running (const char * name)
         int num_processes;
         gchar *command = g_strdup_printf ("pidof %s | wc -l", name);
         FILE *fp = popen(command, "r");
-        fscanf(fp, "%d", &num_processes);
+
+        if (fscanf(fp, "%d", &num_processes) != 1)
+                num_processes = 0;
+
         pclose(fp);
         g_free (command);