client API: fix log buffer overflow case

It just crashed. The prefix and text fields point to static strings in
this case. Oops.

Fixes the issue mentioned in #838.

player/client.c

@@ -751,13 +751,13 @@ mpv_event *mpv_wait_event(mpv_handle *ctx, double timeout)
  event->event_id = MPV_EVENT_LOG_MESSAGE;
  struct mpv_event_log_message *cmsg = talloc_ptrtype(event, cmsg);
  *cmsg = (struct mpv_event_log_message){
- .prefix = talloc_steal(event, msg->prefix),
+ .prefix = msg->prefix,
  .level = mp_log_levels[msg->level],
  .log_level = mp_mpv_log_levels[msg->level],
- .text = talloc_steal(event, msg->text),
+ .text = msg->text,
  };
+ talloc_steal(event, msg);
  event->data = cmsg;
- talloc_free(msg);
  break;
  }
  }