avutil/audio_fifo: add av_warn_unused_result

This one should not trigger any warnings, but will be useful for future
robustness.

Strictly speaking, one could check the size after the call by examining
the structure instead of the return value. Such a use case is highly
unusual, and this commit may be easily reverted if there is a legitimate
need of such use.

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