avcodec/movtextdec: Make tx3g_ptr unsigned

Fixes integer overflow
Fixes: efe937780e95574250dabe07151bdc23/unknown_unknown_351_849_cov_3187578556_shellymanne.mov

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