Fix handling of invalid paths with multiple columns in wxFileName.

Fix handling of invalid paths with multiple columns in wxFileName.

SplitVolume() didn't handle colons in the initial position correctly which
surprised SetPath() and led to accessing an out-of-range string element. Fix
SplitVolume() and also add a check to SetPath() itself as it seems like it
could be called with a path containing the volume only.

Closes #11453.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62677 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775