ffmpeg: preserve NULL ptrs in get_buffer()

Fixes Ticket1260, Ticket1267
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -569,7 +569,7 @@ static int alloc_buffer(InputStream *ist, AVCodecContext *s, 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) || !buf->linesize[1])
+        if ((s->flags & CODEC_FLAG_EMU_EDGE) || !buf->linesize[1] || !buf->base[i])
             buf->data[i] = buf->base[i];
         else
             buf->data[i] = buf->base[i] +