Fix handling of not fully specified min/max size in wxBoxSizer.

Fix handling of not fully specified min/max size in wxBoxSizer.

wxSizerItem::AddBorderToSize() added in r72344 (see #11497) didn't work
correctly as it replaced unspecified (i.e. set to -1) components of wxSize
with the small positive values that did take effect, contrary to the
intention.

Fix it to only adjust the actually set component(s) of wxSize.

Closes #14696.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72586 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775