Fix crash in ffmpeg.c with PIX_FMT_NONE

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -657,6 +657,7 @@ static void choose_pixel_fmt(AVStream *st, AVCodec *codec)
                 break;
         }
         if (*p == -1) {
+            if(st->codec->pix_fmt != PIX_FMT_NONE)
             av_log(NULL, AV_LOG_WARNING,
                    "Incompatible pixel format '%s' for codec '%s', auto-selecting format '%s'\n",
                    av_pix_fmt_descriptors[st->codec->pix_fmt].name,