Fix wxMemoryDC::SetFont(wxNullFont) broken by recent changes

Fix crash in this function introduced by 9e5c8a8027 (Respect bitmap
content scale factor in wxMSW wxMemoryDC, 2022-03-26).

Also add a unit test ensuring that this doesn't get broken again under
MSW, which seems to be the only place where it works.

Closes #22240.

src/msw/dcmemory.cpp

@@ -148,7 +148,8 @@ void wxMemoryDCImpl::SetFont(const wxFont& font)
     // We need to adjust the font size by the ratio between the scale factor we
     // use and the default/global scale factor used when creating fonts.
     wxFont scaledFont = font;
-    scaledFont.WXAdjustToPPI(wxDisplay::GetStdPPI()*m_contentScaleFactor);
+    if ( scaledFont.IsOk() )
+        scaledFont.WXAdjustToPPI(wxDisplay::GetStdPPI()*m_contentScaleFactor);
     wxMSWDCImpl::SetFont(scaledFont);
 }
 

tests/graphics/measuring.cpp

@@ -87,6 +87,14 @@ TEST_CASE("wxMemoryDC::GetTextExtent", "[memdc][text-extent]")
     wxBitmap bmp(100, 100);
     wxMemoryDC memdc(bmp);
     GetTextExtentTester<wxMemoryDC> testMem(memdc);
+
+    // Under MSW, this wxDC should work even without any valid font -- but
+    // this is not the case under wxGTK and probably neither elsewhere, so
+    // restrict this test to that platform only.
+#ifdef __WXMSW__
+    memdc.SetFont(wxNullFont);
+    GetTextExtentTester<wxMemoryDC> testMemNoFont(memdc);
+#endif // __WXMSW__
 }
 
 #if wxUSE_PRINTING_ARCHITECTURE && wxUSE_POSTSCRIPT