Ensure that we never return negative client size.

wxMSW could return negative client size for tiny windows with borders, this
was unexpected and shouldn't happen so explicitly ensure it does not.

Also add a unit test to check that this problem doesn't exist in other ports.

Closes #13184.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67754 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/window.cpp

@@ -1787,6 +1787,15 @@ void wxWindowMSW::DoGetClientSize(int *x, int *y) const
         if ( y )
             *y = rect.bottom;
     }
+
+    // The size of the client window can't be negative but ::GetClientRect()
+    // can return negative size for an extremely small (1x1) window with
+    // borders so ensure that we correct it here as having negative sizes is
+    // completely unexpected.
+    if ( x && *x < 0 )
+        *x = 0;
+    if ( y && *y < 0 )
+        *y = 0;
 }
 
 void wxWindowMSW::DoGetPosition(int *x, int *y) const