avformat/swfdec: check avio_read() return code

Fixes use of uninitialized memory
Fixes msan_uninit-mem_7f90d9cce964_9558_videopop_guitar_300k.swf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
