avformat/av1dec: Check bits left before get_leb128()

Fixes: use of uninitialized value
Fixes: 70872/clusterfuzz-testcase-minimized-ffmpeg_dem_OBU_fuzzer-6005782487826432

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

libavformat/av1dec.c

@@ -326,6 +326,9 @@ static int read_obu_with_size(const uint8_t *buf, int buf_size, int64_t *obu_siz
         skip_bits(&gb, 3);  // extension_header_reserved_3bits
     }
 
+    if (get_bits_left(&gb) < 8)
+        return AVERROR_INVALIDDATA;
+
     *obu_size  = get_leb128(&gb);
     if (*obu_size > INT_MAX)
         return AVERROR_INVALIDDATA;