avcodec/cbs_h2645: Fix showing bits at the end in cbs_read_ue_golomb()

Fixes: Assertion n>0 && n<=25 failed at libavcodec/get_bits.h:375
Fixes: 62617/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5156555663998976

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

libavcodec/cbs_h2645.c

@@ -44,7 +44,7 @@ static int cbs_read_ue_golomb(CodedBitstreamContext *ctx, GetBitContext *gbc,
 
     max_length = FFMIN(get_bits_left(gbc), 32);
 
-    leading_bits = show_bits_long(gbc, max_length);
+    leading_bits = max_length ? show_bits_long(gbc, max_length) : 0;
     if (leading_bits == 0) {
         if (max_length >= 32) {
             av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid ue-golomb code at "