avcodec/cbs_h2645: Treat slices without data as invalid

Slices that end after their header (meaning slices after the header
without any data before the rbsp_stop_one_bit or possibly without any
rbsp_stop_one_bit at all) are invalid and are now dropped. This ensures
that one doesn't run into two asserts in cbs_h2645_write_slice_data().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Fixes: 19629/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_METADATA_fuzzer-5676822528524288
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_h2645.c

@@ -861,6 +861,9 @@ static int cbs_h264_read_nal_unit(CodedBitstreamContext *ctx,
             if (err < 0)
                 return err;
 
+            if (!cbs_h2645_read_more_rbsp_data(&gbc))
+                return AVERROR_INVALIDDATA;
+
             pos = get_bits_count(&gbc);
             len = unit->data_size;
 
@@ -1036,6 +1039,9 @@ static int cbs_h265_read_nal_unit(CodedBitstreamContext *ctx,
             if (err < 0)
                 return err;
 
+            if (!cbs_h2645_read_more_rbsp_data(&gbc))
+                return AVERROR_INVALIDDATA;
+
             pos = get_bits_count(&gbc);
             len = unit->data_size;