Return false on error, not nullptr.

Return false on error, not nullptr.

New in clang 3.6.

Fixes -Winconsistent-missing-override (clang 3.6).

Needed for av_free().

Return false on error, not nullptr.

Fixes -Winconsistent-missing-override (clang 3.6).

New in clang 3.6.

Reduce number of libjack calls.

What JACK expects is already implemented in MPD, just not used.  The
sample format conversion code in the JACK plugin was redundant and
could reduce sound quality.