Also fix crash with GTK+ < 3.8 in wxMimeTypesManager code

GtkIconInfo is not a GObject prior to 3.8, not just in GTK+ 2.
See https://github.com/wxWidgets/wxWidgets/pull/293

src/gtk/mimetype.cpp

@@ -21,19 +21,19 @@
 
 wxString wxGTKMimeTypesManagerImpl::GetIconFromMimeType(const wxString& mime)
 {
+    wxString icon;
     wxGtkString type(g_content_type_from_mime_type(mime.utf8_str()));
 
     wxGtkObject<GIcon> gicon(g_content_type_get_icon(type));
     if ( !gicon )
-        return wxString();
+        return icon;
 
     GtkIconTheme *theme(gtk_icon_theme_get_default());
     if ( !theme )
-        return wxString();
+        return icon;
 
     // Notice that we can't use wxGtkObject here because a special function
-    // needs to be used for freeing this object in GTK+ 2. We should switch to
+    // needs to be used for freeing this object prior to GTK+ 3.8.
-    // using wxGtkObject when support for GTK+ 2 is dropped.
     GtkIconInfo* const giconinfo = gtk_icon_theme_lookup_by_gicon
                                    (
                                         theme,
@@ -42,16 +42,13 @@ wxString wxGTKMimeTypesManagerImpl::GetIconFromMimeType(const wxString& mime)
                                         GTK_ICON_LOOKUP_NO_SVG
                                    );
 
-    wxString icon;
     if ( giconinfo )
     {
         icon = wxString::FromUTF8(gtk_icon_info_get_filename(giconinfo));
 
-#ifdef __WXGTK3__
+        wxGCC_WARNING_SUPPRESS(deprecated-declarations)
-        g_object_unref(giconinfo);
-#else
         gtk_icon_info_free(giconinfo);
-#endif
+        wxGCC_WARNING_RESTORE()
     }
 
     return icon;