jpeg_skip_scanlines(): Avoid NULL + 0 UBSan error This error occurs at the call to (*cinfo->cconvert->color_convert)() in sep_upsample() whenever cinfo->upsample->need_context_rows == TRUE (i.e. whenever h2v2 or h1v2 fancy upsampling is used.) The error is innocuous, since (*cinfo->cconvert->color_convert)() points to a dummy function (noop_convert()) in that case. Fixes #470