avformat/swfdec: Move packet size check before side data allocation

Fixes memleak
Fixes: 9eb9cf5b8c26dd0fa7107ed0348dcc1f/signal_sigabrt_7ffff6ae7c37_8927_f14c2a6ae1ad0bbde2c94f1da50e7074.swf

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