avcodec/hevc_mp4toannexb_bsf: Avoid NULL memcpy()

Fixes: invalid memcpy use
Fixes: 19299/clusterfuzz-testcase-minimized-ffmpeg_BSF_HEVC_MP4TOANNEXB_fuzzer-5169193398042624

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

libavcodec/hevc_mp4toannexb_bsf.c

@@ -164,7 +164,7 @@ static int hevc_mp4toannexb_filter(AVBSFContext *ctx, AVPacket *out)
         if (ret < 0)
             goto fail;
 
-        if (add_extradata)
+        if (extra_size)
             memcpy(out->data + prev_size, ctx->par_out->extradata, extra_size);
         AV_WB32(out->data + prev_size + extra_size, 1);
         bytestream2_get_buffer(&gb, out->data + prev_size + 4 + extra_size, nalu_size);