ffmpeg: Fix image allocation.

This probably fixes some of the use of uninitialized issues valgrind shows in fate.
It might also fix other issues.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -510,7 +510,7 @@ static int alloc_buffer(AVCodecContext *s, InputStream *ist, FrameBuffer **pbuf)
     for (i = 0; i < FF_ARRAY_ELEMS(buf->data); i++) {
         const int h_shift = i==0 ? 0 : h_chroma_shift;
         const int v_shift = i==0 ? 0 : v_chroma_shift;
-        if (s->flags & CODEC_FLAG_EMU_EDGE)
+        if ((s->flags & CODEC_FLAG_EMU_EDGE) || !buf->linesize[1])
             buf->data[i] = buf->base[i];
         else
             buf->data[i] = buf->base[i] +