mss2: Fix buffer overflow.

Reported as https://trac.mplayerhq.hu/ticket/2264 but have
not been able to reproduce with FFmpeg-only.
I have no idea what coded_height is used for here exactly,
so this might not be the best fix.
Fixes the following chain of events:
ff_mss12_decode_init sets coded_height while not setting height.
ff_mpv_decode_init then copies coded_height into MpegEncContext height.
This is then used by init_context_frame to allocate the data structures.
However the wmv9rects are validated/initialized based on avctx->height, not
avctx->coded_height.
Thus the decode_wmv9 function will try to decode a larger video that we
allocated data structures for, causing out-of-bounds writes.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/mss12.c

@@ -581,8 +581,8 @@ av_cold int ff_mss12_decode_init(MSS12Context *c, int version,
         return AVERROR_INVALIDDATA;
     }
 
-    avctx->coded_width  = AV_RB32(avctx->extradata + 20);
-    avctx->coded_height = AV_RB32(avctx->extradata + 24);
+    avctx->coded_width  = FFMAX(AV_RB32(avctx->extradata + 20), avctx->width);
+    avctx->coded_height = FFMAX(AV_RB32(avctx->extradata + 24), avctx->height);
     if (avctx->coded_width > 4096 || avctx->coded_height > 4096) {
         av_log(avctx, AV_LOG_ERROR, "Frame dimensions %dx%d too large",
                avctx->coded_width, avctx->coded_height);