st-theme: ref items in custom stylesheets list

st-theme stores some loaded stylesheets in a custom
stylesheets list.  When removing items from this list
it unrefs them, but when adding items to the list it
neglects to ref them.  This means that under certain
circumstances the list will contain items that have
already been freed.

https://bugzilla.gnome.org/show_bug.cgi?id=632477

src/st/st-theme.c

@@ -320,6 +320,7 @@ st_theme_load_stylesheet (StTheme    *theme,
     return FALSE;
 
   insert_stylesheet (theme, path, stylesheet);
+  cr_stylesheet_ref (stylesheet);
   theme->custom_stylesheets = g_slist_prepend (theme->custom_stylesheets, stylesheet);
 
   return TRUE;