avcodec/vp3: Check width to avoid assertion failure

Fixes: Assertion failure on x86-32
       av_assert2(block_w * sizeof(pixel) <= FFABS(buf_linesize)); in ff_emulated_edge_mc()
Fixes: 39641/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THEORA_fuzzer-5925660741206016

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <[email protected]>
(cherry picked from commit dab1cd2)
Signed-off-by: Michael Niedermayer <[email protected]>

libavcodec/vp3.c

@@ -2332,6 +2332,8 @@ static av_cold int vp3_decode_init(AVCodecContext *avctx)
     s->avctx  = avctx;
     s->width  = FFALIGN(avctx->coded_width, 16);
     s->height = FFALIGN(avctx->coded_height, 16);
+    if (s->width < 18)
+        return AVERROR_PATCHWELCOME;
     if (avctx->codec_id != AV_CODEC_ID_THEORA)
         avctx->pix_fmt = AV_PIX_FMT_YUV420P;
     avctx->chroma_sample_location = AVCHROMA_LOC_CENTER;
@@ -2978,7 +2980,9 @@ static int theora_decode_header(AVCodecContext *avctx, GetBitContext *gb)
     /* sanity check */
     if (av_image_check_size(visible_width, visible_height, 0, avctx) < 0 ||
         visible_width  + offset_x > s->width ||
-        visible_height + offset_y > s->height) {
+        visible_height + offset_y > s->height ||
+        visible_width < 18
+    ) {
         av_log(avctx, AV_LOG_ERROR,
                "Invalid frame dimensions - w:%d h:%d x:%d y:%d (%dx%d).\n",
                visible_width, visible_height, offset_x, offset_y,
@@ -3024,6 +3028,8 @@ static int theora_decode_header(AVCodecContext *avctx, GetBitContext *gb)
     } else
         avctx->pix_fmt = AV_PIX_FMT_YUV420P;
 
+    if (s->width < 18)
+        return AVERROR_PATCHWELCOME;
     ret = ff_set_dimensions(avctx, s->width, s->height);
     if (ret < 0)
         return ret;