Don't enter an infinite loop if a spacer with min size of -1 is used.

Don't enter an infinite loop if a spacer with min size of -1 is used.

Sizer layout algorithm broke down if min size of an item happened to be -1,
i.e. the same value as we use as a sentinel for indicating that the min size
hasn't been fixed yet. It doesn't make much sense for min size to be negative
in the first place but currently this can happen at least for spacers so deal
with it here by ensuring that the min size we use is positive.

Closes #11842.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63735 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775