Fix assert when using year or decade view in Windows 7 calendar control.

Fix assert when using year or decade view in Windows 7 calendar control.

Windows 7 native calendar control is capable of showing more than 3 months
simultaneously in its year or decade view which is shown when the user zooms
out of the month view by double clicking the control header. This resulted in
an assert failure in the code, update it to simply not do anything in this
view.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63561 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775