ff_vvc_palette_escape_val() can return AVERROR in which case the
coeff*scale will overflow.
Fixes: runtime error: signed integer overflow: -1094995529 * 6528 cannot
be represented in type 'int'
Fixes: 435225406/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VVC_fuzzer-5118570024730624
Found-by: OSS-Fuzz
Signed-off-by: Kacper Michajłow <kasper93@gmail.com>