avcodec/g2meet: Check R/G/B values in epic_decode_pixel_pred()

Fixes: asan_double-free_d34593_861_smp3.wmv

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
