avcodec/wmaenc: Check ff_wma_init() for failure

Fixes null pointer dereference
Fixes: c4faf8280ba366bf00a79d425f2910a8/signal_sigsegv_1f96477_5177_1448ba7e4125faceb966f44ceb69abfa.qcp
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
