svq1dec: check that the reference frame has the same dimensions as the current one

They can be different if the last keyframe failed to decode correctly.
Fixes possible invalid reads in such a case.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
(cherry picked from commit b1bb8fb860)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/svq1dec.c

@@ -692,7 +692,8 @@ static int svq1_decode_frame(AVCodecContext *avctx, void *data,
         } else {
             /* delta frame */
             uint8_t *previous = s->prev->data[i];
-            if (!previous) {
+            if (!previous ||
+                s->prev->width != s->width || s->prev->height != s->height) {
                 av_log(avctx, AV_LOG_ERROR, "Missing reference frame.\n");
                 result = AVERROR_INVALIDDATA;
                 goto err;