lavc/utils: Do not require dimensions for PNG.

pngenc doesnt need them and mplayer doesnt set them.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -1000,7 +1000,9 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
         } else if (avctx->channel_layout) {
             avctx->channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
         }
-        if(avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
+        if(avctx->codec_type == AVMEDIA_TYPE_VIDEO &&
+           avctx->codec_id != AV_CODEC_ID_PNG // For mplayer
+        ) {
             if (avctx->width <= 0 || avctx->height <= 0) {
                 av_log(avctx, AV_LOG_ERROR, "dimensions not set\n");
                 ret = AVERROR(EINVAL);