avformat/oggdec: Check buf before copying data in to it

Fixes null pointer dereference
Fixes: aace024653cc62947336b86f8de812ab_signal_sigsegv_a0500f_343_WobblyWindowsIntro.ogg with memlimit 262144

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