Avoid integer overflow/wraparound in wxString::Mid().

Don't compare nLength with "nFirst + nCount" as this could wrap around.

Compare nCount with maximal allowed count, after ensuring that nFirst itself
is valid first, instead.

Closes #16572.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77725 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775