lavc: dont call set_dimensions() on h264 codec init.

This fixes ffprobe showing an incorrect width with
http://panda-test-harness-videos.s3.amazonaws.com/panda.mp4

Idea-by: Joakim Plate <elupus@ecce.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -575,10 +575,13 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, AVCodec *codec, AVD
     if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
         goto free_and_end;
 
+    //We only call avcodec_set_dimensions() for non h264 codecs so as not to overwrite previously setup dimensions
+    if(!( avctx->coded_width && avctx->coded_height && avctx->width && avctx->height && avctx->codec_id == CODEC_ID_H264)){
     if(avctx->coded_width && avctx->coded_height)
         avcodec_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);
     else if(avctx->width && avctx->height)
         avcodec_set_dimensions(avctx, avctx->width, avctx->height);
+    }
 
     if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)
         && (  av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx) < 0