handle malloc failures in ff_get_wav_header

ff_get_wav_header is reading data from a WAVE file and then uses it
(without validation) to malloc a buffer. It then proceeded to read
data into the buffer, without verifying that the allocation succeeded.

To address this, change ff_get_wav_header to return an error if
allocation failed, and adapted all calling code to handle that error.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>