#27 Zoom center can crash application

[Stephen Denham]

If you select zoom center, click once in the main
drawing window and then click again in exactly the same
place (i.e. without moving the mouse) then the
application crashes.

I think this is a divide by zero error in vectorize.cpp
(void LX_ZoomCenter(AD_DB_HANDLE dwghandle). A
check is needed to make sure the anchor points and end
points are sufficiently different.