avcodec/hevc_sei: Fix integer overflows in decode_nal_sei_message()

Fixes: signed integer overflow: 2147483520 + 255 cannot be represented in type 'int'
Fixes: 4554/clusterfuzz-testcase-minimized-4843714515042304

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 991ef6e5b9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevc_sei.c

@@ -344,11 +344,15 @@ static int decode_nal_sei_message(HEVCContext *s)
     av_log(s->avctx, AV_LOG_DEBUG, "Decoding SEI\n");
 
     while (byte == 0xFF) {
+        if (get_bits_left(gb) < 16 || payload_type > INT_MAX - 255)
+            return AVERROR_INVALIDDATA;
         byte          = get_bits(gb, 8);
         payload_type += byte;
     }
     byte = 0xFF;
     while (byte == 0xFF) {
+        if (get_bits_left(gb) < 8 + 8LL*payload_size)
+            return AVERROR_INVALIDDATA;
         byte          = get_bits(gb, 8);
         payload_size += byte;
     }