[libhandy] stackable-box: Null check visible child in find_swipeable_child()

[Adrien Plazas <aplazas src gnome org>]

commit 2a553156dce72cd3ad1daa29b509ada1c9e33f5b
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Jun 18 14:51:07 2021 +0500

    stackable-box: Null check visible child in find_swipeable_child()
    
    Fixes https://gitlab.gnome.org/GNOME/libhandy/-/issues/444

 src/hdy-stackable-box.c | 4 ++++
 tests/test-deck.c       | 6 ++++++
 tests/test-leaflet.c    | 6 ++++++
 3 files changed, 16 insertions(+)
---
diff --git a/src/hdy-stackable-box.c b/src/hdy-stackable-box.c
index 4d003c0e..7ebeb66f 100644
--- a/src/hdy-stackable-box.c
+++ b/src/hdy-stackable-box.c
@@ -1197,6 +1197,10 @@ find_swipeable_child (HdyStackableBox        *self,
   HdyStackableBoxChildInfo *child = NULL;
 
   children = g_list_find (self->children, self->visible_child);
+
+  if (children == NULL)
+    return NULL;
+
   do {
     children = (direction == HDY_NAVIGATION_DIRECTION_BACK) ? children->prev : children->next;
 
diff --git a/tests/test-deck.c b/tests/test-deck.c
index 9a1fd920..51ef5129 100644
--- a/tests/test-deck.c
+++ b/tests/test-deck.c
@@ -54,6 +54,12 @@ test_hdy_deck_navigate (void)
   deck = HDY_DECK (hdy_deck_new ());
   g_assert_nonnull (deck);
 
+  result = hdy_deck_navigate (deck, HDY_NAVIGATION_DIRECTION_BACK);
+  g_assert_false (result);
+
+  result = hdy_deck_navigate (deck, HDY_NAVIGATION_DIRECTION_FORWARD);
+  g_assert_false (result);
+
   for (i = 0; i < 2; i++) {
     children[i] = gtk_label_new ("");
     g_assert_nonnull (children[i]);
diff --git a/tests/test-leaflet.c b/tests/test-leaflet.c
index 506c0a44..f06ad9a3 100644
--- a/tests/test-leaflet.c
+++ b/tests/test-leaflet.c
@@ -66,6 +66,12 @@ test_hdy_leaflet_navigate (void)
   leaflet = HDY_LEAFLET (hdy_leaflet_new ());
   g_assert_nonnull (leaflet);
 
+  result = hdy_leaflet_navigate (leaflet, HDY_NAVIGATION_DIRECTION_BACK);
+  g_assert_false (result);
+
+  result = hdy_leaflet_navigate (leaflet, HDY_NAVIGATION_DIRECTION_FORWARD);
+  g_assert_false (result);
+
   for (i = 0; i < 3; i++) {
     children[i] = gtk_label_new ("");
     g_assert_nonnull (children[i]);