Attempt to fix KZoneAllocator issue

kcompletion.p_h: Make the static KZoneAllocator member of KCompTreeNode
a shared pointer so external users can hold a reference to it.

kcompletion.cpp: Hold a reference to KCompTreeNode's KZoneAllocator
instance so we avoid deleting the KZoneAllocator too early. See attached
bug report for possible causes. (Hint: It crashes on Windows because
~KZoneAllocator is called to early.)

kallocator.cpp: Use printf instead of qDebug(), because this code path
code might be called very late during destruction and qDebug() will
crash deep inside Qt.

Also see discussion:
http://lists.kde.org/?l=kde-devel&m=138583383708455&w=1

BUG: 327599
CCBUG: 243375
REVIEW: 114715