Hugo Lefeuvre authored Sep 26, 2018 and Daniel Boles committed Oct 07, 2018

The gtk_stack_snapshot_slide() function dereferences the
last_visible_child pointer without proper != NULL ckeck. This might
result in NULL pointer dereference and crash if last_visible_child is
invalid.

Add a != NULL check before dereferencing the pointer.

cherry-picked from !361