avcodec/wmaenc: Check input for finiteness

Fixes out of array read
Fixes: 1b79b985cdf860ffa228c00ee5497051/signal_sigsegv_1f99d24_315_f0f1fdb451264b1138e9c24eb7d49181.wv

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