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

Fixes: out of array access
Fixes: 29202/clusterfuzz-testcase-minimized-ffmpeg_dem_KUX_fuzzer-5112845840809984

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

@@ -41,6 +41,8 @@
 
 #define RESYNC_BUFFER_SIZE (1<<20)
 
+#define MAX_DEPTH 16      ///< arbitrary limit to prevent unbounded recursion
+
 typedef struct FLVContext {
     const AVClass *class; ///< Class for private options.
     int trust_metadata;   ///< configure streams according onMetaData
@@ -493,6 +495,9 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
     double num_val;
     amf_date date;
 
+    if (depth > MAX_DEPTH)
+        return AVERROR_PATCHWELCOME;
+
     num_val  = 0;
     ioc      = s->pb;
     if (avio_feof(ioc))