player: don't signal an error if --stream-dump is used

The player thinks an error happened because no audio or video was played
after finishing the file, but this obviously makes no sense with stream
dumping. (error_playing follows the client API convention that negative
values are errors.)

player/loadfile.c

@@ -966,6 +966,7 @@ static void play_current_file(struct MPContext *mpctx)
 
     if (opts->stream_dump && opts->stream_dump[0]) {
         stream_dump(mpctx);
+        mpctx->error_playing = 1;
         goto terminate_playback;
     }