Return correct font from gtk_font_chooser_widget_find_font

Return correct font from gtk_font_chooser_widget_find_font

Commit 30a1c4ab fixed several memleaks including one in
gtk_font_chooser_widget_find_font.

However, the fix causes one extra call to gtk_tree_model_iter_next()
after finding the font we look for (ie pango_font_description_equal
returns TRUE): the 'increment' part of the for loop
(gtk_tree_model_iter_next) is run before the 'exit condition' of the for
loop is evaluated.

This commit reverts this part of commit 30a1c4ab and adds an extra
call to pango_font_description_free in order to fix the leak.

https://bugzilla.gnome.org/show_bug.cgi?id=739111