avcodec/cbs_vp8: fix GetBitContext setup

Fixes: abort()
Fixes: 64232/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5417957987319808

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: James Almer <jamrial@gmail.com>

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

libavcodec/cbs_vp8.c

@@ -307,7 +307,7 @@ static int cbs_vp8_read_unit(CodedBitstreamContext *ctx,
     frame = unit->content;
 
     // Create GetBitContext for uncompressed header.
-    err = init_get_bits8_le(&gbc, unit->data, 8 * unit->data_size);
+    err = init_get_bits8_le(&gbc, unit->data, unit->data_size);
     if (err < 0)
         return err;