avcodec/ass_split: Fix null pointer dereference in ff_ass_style_get()

Fixes: 55d71971da50365d542ed14b65565fe1/signal_sigsegv_4765a4_8499_f146af090a94f591d6254515c7700ef5.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
