Use-after-free in HdyStackableBox
Found by valgrind:
==105328== Invalid read of size 8
==105328== at 0xAD2AE66: hdy_stackable_box_get_visible_child (hdy-stackable-box.c:952)
==105328== by 0xAD2DD4A: hdy_stackable_box_remove (hdy-stackable-box.c:2313)
==105328== by 0xAD03C4A: hdy_deck_remove (hdy-deck.c:697)
==105328== by 0x554DDB1: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1910)
==105328== by 0x5548F45: g_type_class_meta_marshalv (gclosure.c:1034)
==105328== by 0x5548A98: _g_closure_invoke_va (gclosure.c:873)
==105328== by 0x55664F0: g_signal_emit_valist (gsignal.c:3406)
==105328== by 0x5567782: g_signal_emit (gsignal.c:3553)
==105328== by 0x4B1D569: gtk_container_remove (gtkcontainer.c:1906)
==105328== by 0x4E4595B: gtk_widget_dispose (gtkwidget.c:12151)
==105328== by 0x4AAC36A: gtk_box_dispose (gtkbox.c:240)
==105328== by 0x48BAF33: ephy_pages_view_dispose (ephy-pages-view.c:115)
==105328== Address 0x446ffb90 is 0 bytes inside a block of size 64 free'd
==105328== at 0x48430E4: free (vg_replace_malloc.c:755)
==105328== by 0x5601F8E: g_free (gmem.c:199)
==105328== by 0xAD295DD: free_child_info (hdy-stackable-box.c:175)
==105328== by 0xAD295FF: glib_autoptr_clear_HdyStackableBoxChildInfo (hdy-stackable-box.c:178)
==105328== by 0xAD2961D: glib_autoptr_cleanup_HdyStackableBoxChildInfo (hdy-stackable-box.c:178)
==105328== by 0xAD2DDF8: hdy_stackable_box_remove (hdy-stackable-box.c:2301)
==105328== by 0xAD03C4A: hdy_deck_remove (hdy-deck.c:697)
==105328== by 0x554DDB1: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1910)
==105328== by 0x5548F45: g_type_class_meta_marshalv (gclosure.c:1034)
==105328== by 0x5548A98: _g_closure_invoke_va (gclosure.c:873)
==105328== by 0x55664F0: g_signal_emit_valist (gsignal.c:3406)
==105328== by 0x5567782: g_signal_emit (gsignal.c:3553)
==105328== Block was alloc'd at
==105328== at 0x4845464: calloc (vg_replace_malloc.c:1117)
==105328== by 0x5601EA4: g_malloc0 (gmem.c:136)
==105328== by 0x5602187: g_malloc0_n (gmem.c:368)
==105328== by 0xAD2F932: hdy_stackable_box_insert_child_after (hdy-stackable-box.c:2912)
==105328== by 0xAD2DC1D: hdy_stackable_box_add (hdy-stackable-box.c:2289)
==105328== by 0xAD03C11: hdy_deck_add (hdy-deck.c:690)
==105328== by 0x554DDB1: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1910)
==105328== by 0x5548F45: g_type_class_meta_marshalv (gclosure.c:1034)
==105328== by 0x5548A98: _g_closure_invoke_va (gclosure.c:873)
==105328== by 0x55664F0: g_signal_emit_valist (gsignal.c:3406)
==105328== by 0x5567782: g_signal_emit (gsignal.c:3553)
==105328== by 0x4B1D3D9: gtk_container_add (gtkcontainer.c:1876)