lavc: Relax API strictness in avcodec_decode_audio3 with a custom get_buffer()

Do not fail audio decoding with avcodec_decode_audio3 if user has set a
custom get_buffer. Strictly speaking, this was never allowed by the API,
but it seems that some software packages did so anyways. In order to
unbreak applications (cf. http://bugs.debian.org/655890), this change
clarifies the API and overrides the custom get_buffer() with the defaults.

This change is inspired by a similar
commit (c3846e3ebab610be691adb8b40d376dc2f675dc4) in FFmpeg.

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
