glocalfilemonitor: Fix data race in local file monitor

Ensure that source is attached to the context before it migth be used
from another thread, since otherwise operation on source are
unsynchronized and not thread-safe.

In particular there was a data race between g_source_attach and
g_source_set_ready_time (used from g_file_monitor_source_handle_event).

gio/glocalfilemonitor.c

@@ -788,11 +788,11 @@ g_local_file_monitor_start (GLocalFileMonitor *local_monitor,
 #endif
     }
 
+  g_source_attach ((GSource *) source, context);
+
   G_LOCAL_FILE_MONITOR_GET_CLASS (local_monitor)->start (local_monitor,
                                                          source->dirname, source->basename, source->filename,
                                                          source);
-
-  g_source_attach ((GSource *) source, context);
 }
 
 static void