HR fix of bug 255668 - NullPointerException at org.netbeans.modules.editor.lib2.view.DocumentViewOp.mouseWheelMoved

http://netbeans.org/issues/show_bug.cgi?id=255668

Using mouse wheel in some editor panes generates NPE. Regular editor 
windows are not affected but e.g. left editor in Diff view is. The 
problem was caused by some changes in overriding of default mouse wheel 
handler due to editor's text font zooming feature (Alt+mouse-wheel).
The fix makes the overriding more robust.

http://hg.netbeans.org/main-silver/rev/afdd03c528b2

Thanks,
Mila