core: don't allow double-scheduled callbacks

If the modem becomes invalid (it crashes and resets, for example)
the callback-info's modem_destroyed_cb() function will handle
cleanup.  Buf if the callback-info's callback does more work than
just returning the result (like simple_state_machine) it could
double-schedule the callback.  Don't let that happen.

We need better modem-removal handling, but this fixes a crash for
now.

src/mm-callback-info.c

@@ -67,6 +67,7 @@ callback_info_done (gpointer user_data)
     MMCallbackInfo *info = (MMCallbackInfo *) user_data;
 
     info->pending_id = 0;
+    info->called = TRUE;
 
     if (info->invoke_fn && info->callback)
         info->invoke_fn (info);
@@ -94,6 +95,7 @@ mm_callback_info_schedule (MMCallbackInfo *info)
 {
     g_return_if_fail (info != NULL);
     g_return_if_fail (info->pending_id == 0);
+    g_return_if_fail (info->called == FALSE);
 
     info->pending_id = g_idle_add_full (G_PRIORITY_DEFAULT_IDLE, callback_info_do, info, callback_info_done);
 }

src/mm-callback-info.h

@@ -28,6 +28,7 @@ struct _MMCallbackInfo {
 
     MMCallbackInfoInvokeFn invoke_fn;
     GCallback callback;
+    gboolean called;
 
     gpointer user_data;
     GError *error;