avcodec/shorten: Sanity check nmeans

Fixes: OOM
Fixes: 8195/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5179785826271232

The reference software appears to use longs for 32bits and it uses int for nmeans
hinting that the intended maximum size was not 32bit.

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 d91a0b503d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -374,6 +374,10 @@ static int read_header(ShortenContext *s)
             return AVERROR_INVALIDDATA;
         }
         s->nmean = get_uint(s, 0);
+        if (s->nmean > 32768U) {
+            av_log(s->avctx, AV_LOG_ERROR, "nmean is: %d\n", s->nmean);
+            return AVERROR_INVALIDDATA;
+        }
 
         skip_bytes = get_uint(s, NSKIPSIZE);
         if ((unsigned)skip_bytes > get_bits_left(&s->gb)/8) {