Fix using invalid HFONT in wxGetTextMetrics()

If the window has no valid font, GetFont() returns a temporary font.
Extend this font lifetime, so the HFONT remains valid till the end of
the function.

src/msw/window.cpp

@@ -7421,7 +7421,12 @@ static TEXTMETRIC wxGetTextMetrics(const wxWindowMSW *win)
 
 #if !wxDIALOG_UNIT_COMPATIBILITY
     // and select the current font into it
-    HFONT hfont = GetHfontOf(win->GetFont());
+
+    // Note that it's important to extend the lifetime of the possibly
+    // temporary wxFont returned by GetFont() to ensure that its HFONT remains
+    // valid.
+    const wxFont& f(win->GetFont());
+    HFONT hfont = GetHfontOf(f);
     if ( hfont )
     {
         hfont = (HFONT)::SelectObject(hdc, hfont);