avcodec/sheervideo: Check input buffer size before allocating and decoding

Fixes: Timeout
Fixes: 1858/clusterfuzz-testcase-minimized-6450473802399744

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/sheervideo.c

@@ -3105,6 +3105,11 @@ static int decode_frame(AVCodecContext *avctx,
         return AVERROR_PATCHWELCOME;
     }
 
+    if (avpkt->size < 20 + avctx->width * avctx->height / 16) {
+        av_log(avctx, AV_LOG_ERROR, "Input packet too small\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     if (s->format != format) {
         if (ret < 0)
             return ret;