avcodec/eatgq: Check bytestream2_get_buffer() for failure

Fixes: Use of uninitialized memory
Fixes: 71546/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EATGQ_fuzzer-5607656650244096

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 4949e34d96)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/eatgq.c

@@ -178,7 +178,8 @@ static int tgq_decode_mb(TgqContext *s, GetByteContext *gbyte,
             dc[4] = bytestream2_get_byte(gbyte);
             dc[5] = bytestream2_get_byte(gbyte);
         } else if (mode == 6) {
-            bytestream2_get_buffer(gbyte, dc, 6);
+            if (bytestream2_get_buffer(gbyte, dc, 6) != 6)
+                return AVERROR_INVALIDDATA;
         } else if (mode == 12) {
             for (i = 0; i < 6; i++) {
                 dc[i] = bytestream2_get_byte(gbyte);