Don't write spurious </font> to wxRichText HTML output

The opening <font> tag was only written to the output if the style had a
non-default font, but the matching closing tag wasn't guarded by the
same check.

Do add the check now to ensure that the tags are always balanced.

Closes #18157.

src/richtext/richtexthtml.cpp

@@ -166,7 +166,8 @@ bool wxRichTextHTMLHandler::DoSaveFile(wxRichTextBuffer *buffer, wxOutputStream&
 
         CloseLists(-1, str);
 
-        str << wxT("</font>");
+        if (currentParaStyle.HasFont())
+            str << wxT("</font>");
 
         if ((GetFlags() & wxRICHTEXT_HANDLER_NO_HEADER_FOOTER) == 0)
             str << wxT("</body></html>");