avcodec/faxcompr: Add missing runs check in decode_uncompressed()

Fixes out of array access
Fixes: 54e488b9da4abbceaf405d6492515697/asan_heap-oob_32769b0_160_a8755eb08ee8f9579348501945a33955.TIF

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