avcodec/vp3: Clear context on reinitialization failure

Fixes null pointer dereference
Fixes: 1536b9b096a8f95b742bae9d3d761cc6/signal_sigsegv_294aaed_2039_8d1797aeb823ea43858d0fa45c9eb899.ogv

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