avformat/flvdec: Check array entry number

Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 30209/clusterfuzz-testcase-minimized-ffmpeg_dem_FLV_fuzzer-5724831658147840

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 b5d8fe1c87)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -875,6 +875,8 @@ static int amf_skip_tag(AVIOContext *pb, AMFDataType type, int depth)
         parse_name = 0;
     case AMF_DATA_TYPE_MIXEDARRAY:
         nb = avio_rb32(pb);
+        if (nb < 0)
+            return AVERROR_INVALIDDATA;
     case AMF_DATA_TYPE_OBJECT:
         while(!pb->eof_reached && (nb-- > 0 || type != AMF_DATA_TYPE_ARRAY)) {
             if (parse_name) {