aout_OutputNextBuffer: move both FIFOs atomically and clean up
aout_OutputNextBuffer: move both FIFOs atomically and clean up

This avoids racing with decoder thread (audio input & mixer) w.r.t. the
FIFO timestamps.

shyamalschandra/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,xkfz007/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.2,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,xkfz007/vlc