avcodec/vp3: check current_frame before accessing it

Fixes null pointer dereference
Fixes: b15eb06e0111e94bc59123c86db7aff9_signal_sigsegv_a0500f_45_320vp3.nsv with allocation limit  536870912

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