lavc/wma: add av_warn_unused_result to ff_wma_init

May help to prevent incidents like 19e456d48c90a1e3ceeb9e6241383384cc73dfdf.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
