fix for [#33803], error was caused by sloppy coding in r53487, converting trackpad to wheel events.

if you moved your mouse fast over a button the event would get converted to a wheel, even if the input event wasnt a MOUSEPAN event.

When Alt was held this was noticable because Alt+Wheel changes button values.

added an assert to avoid this happening again.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54246 954f8c5b-7b00-dc11-b283-0030488c597c