Fix leaks in dynamic menus

Fixes two leaks:
```
10,463 (48 direct, 10,415 indirect) bytes in 1 blocks are definitely lost in loss record 34,138 of 34,479
at 0x484386F: malloc (vg_replace_malloc.c:393)
by 0x4980168: g_malloc (gmem.c:130)
by 0x4997AB5: g_slice_alloc (gslice.c:1074)
by 0x49980EC: g_slice_alloc0 (gslice.c:1100)
by 0x4DDF04B: g_type_create_instance (gtype.c:1913)
by 0x4DC4C1F: g_object_new_internal (gobject.c:2228)
by 0x4DC6247: g_object_new_with_properties (gobject.c:2391)
by 0x4DC6FF0: g_object_new (gobject.c:2037)
by 0x1CA978CF: purple_demo_protocol_get_menu (purpledemoprotocolactions.c:750)
by 0x4D2DB1F: purple_protocol_actions_get_menu (purpleprotocolactions.c:110)
by 0x489F74C: pidgin_accounts_enabled_menu_get_item_links (pidginaccountsenabledmenu.c:279)
by 0x5817C3F: g_menu_model_real_get_item_link (gmenumodel.c:397)
```
and
```
2,051 (144 direct, 1,907 indirect) bytes in 3 blocks are definitely lost in loss record 33,018 of 34,479
at 0x484386F: malloc (vg_replace_malloc.c:393)
by 0x4980168: g_malloc (gmem.c:130)
by 0x4997AB5: g_slice_alloc (gslice.c:1074)
by 0x49980EC: g_slice_alloc0 (gslice.c:1100)
by 0x4DDF04B: g_type_create_instance (gtype.c:1913)
by 0x4DC4C1F: g_object_new_internal (gobject.c:2228)
by 0x4DC6247: g_object_new_with_properties (gobject.c:2391)
by 0x4DC6FF0: g_object_new (gobject.c:2037)
by 0x581E9EE: g_menu_item_new_section (gmenu.c:1256)
by 0x48B465F: pidgin_plugins_menu_get_item_links (pidginpluginsmenu.c:204)
by 0x5817C3F: g_menu_model_real_get_item_link (gmenumodel.c:397)
by 0x5124B78: gtk_menu_tracker_add_items (gtkmenutracker.c:313)
```

Testing Done:
Ran in `valgrind --leak-check=full` and confirmed that the leak was gone.

Reviewed at https://reviews.imfreedom.org/r/2298/