avformat/flvdec: Check for nesting depth in amf_skip_tag()

Fixes: out of array access
Fixes: 29440/clusterfuzz-testcase-minimized-ffmpeg_dem_KUX_fuzzer-5985279812960256.fuzz

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

libavformat/flvdec.c

@@ -842,10 +842,13 @@ static void clear_index_entries(AVFormatContext *s, int64_t pos)
     }
 }
 
-static int amf_skip_tag(AVIOContext *pb, AMFDataType type)
+static int amf_skip_tag(AVIOContext *pb, AMFDataType type, int depth)
 {
     int nb = -1, ret, parse_name = 1;
 
+    if (depth > MAX_DEPTH)
+        return AVERROR_PATCHWELCOME;
+
     switch (type) {
     case AMF_DATA_TYPE_NUMBER:
         avio_skip(pb, 8);
@@ -870,7 +873,7 @@ static int amf_skip_tag(AVIOContext *pb, AMFDataType type)
                 }
                 avio_skip(pb, size);
             }
-            if ((ret = amf_skip_tag(pb, avio_r8(pb))) < 0)
+            if ((ret = amf_skip_tag(pb, avio_r8(pb), depth + 1)) < 0)
                 return ret;
         }
         break;
@@ -914,7 +917,7 @@ static int flv_data_packet(AVFormatContext *s, AVPacket *pkt,
             else
                 break;
         } else {
-            if ((ret = amf_skip_tag(pb, type)) < 0)
+            if ((ret = amf_skip_tag(pb, type, 0)) < 0)
                 goto skip;
         }
     }