avformat/shortendec: Check k in probe

Fixes: Assertion failure
Fixes: 17640/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5708767475269632

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ea770eb5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/shortendec.c

@@ -40,12 +40,18 @@ static int shn_probe(AVProbeData *p)
         channels = get_ur_golomb_shorten(&gb, 0);
         blocksize = 256;
     } else {
-        int k;
+        unsigned k;
         k = get_ur_golomb_shorten(&gb, 2);
+        if (k > 31)
+            return 0;
         internal_ftype = get_ur_golomb_shorten(&gb, k);
         k = get_ur_golomb_shorten(&gb, 2);
+        if (k > 31)
+            return 0;
         channels = get_ur_golomb_shorten(&gb, k);
         k = get_ur_golomb_shorten(&gb, 2);
+        if (k > 31)
+            return 0;
         blocksize = get_ur_golomb_shorten(&gb, k);
     }