Initialize audioOutput->data with NULL in jack_initDriver().
Previously, this was never initialized, although the other functions
relied on it being NULL prior to jack_openDevice().

This patch addresses bug 0001641[1].  In contrast to the patch provided
by the bug reporter, it moves the initialization before the "!param"
check.

[1] - http://musicpd.org/mantis/view.php?id=1641

git-svn-id: https://svn.musicpd.org/mpd/trunk@7375 09075e82-0dd4-0310-85a5-a0d7c8717e4f