avformat/segafilm: Only add index entries when the stream exists

Fixes null pointer dereference
Fixes: cb02dfb163ac833c04cace3d7e35b160/signal_sigsegv_e55c49_6326_8f25619fc86a00b303c57b7778baf70a.cpk

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