avcodec/exr: Check for remaining bits in huf_unpack_enc_table()

Fixes: Timeout
Fixes: 67645/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-6308760977997824

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 589fa8a027)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/exr.c

@@ -334,7 +334,10 @@ static int huf_unpack_enc_table(GetByteContext *gb,
         return ret;
 
     for (; im <= iM; im++) {
-        uint64_t l = freq[im] = get_bits(&gbit, 6);
+        uint64_t l;
+        if (get_bits_left(&gbit) < 6)
+            return AVERROR_INVALIDDATA;
+        l = freq[im] = get_bits(&gbit, 6);
 
         if (l == LONG_ZEROCODE_RUN) {
             int zerun = get_bits(&gbit, 8) + SHORTEST_LONG_RUN;