sequence: add g_sequence_is_empty()

This function provides an O(1) check to determine if a sequence is empty.
Compare this to the two following alternatives to perform the same check.

O(h):  if (0 == g_sequence_get_length (seq))
O(2h): if (g_sequence_get_begin_iter(seq) == g_sequence_get_end_iter(seq))

Where `h' is the height of the tree.

https://bugzilla.gnome.org/show_bug.cgi?id=756316

glib/gsequence.c

@@ -1243,6 +1243,26 @@ g_sequence_get_length (GSequence *seq)
   return node_get_length (seq->end_node) - 1;
 }
 
+/**
+ * g_sequence_is_empty:
+ * @seq: a #GSequence
+ *
+ * Returns %TRUE if the sequence contains zero items.
+ *
+ * This function is functionally identical to checking the result of
+ * g_sequence_get_length() being equal to zero. However this function is
+ * implemented in O(1) running time.
+ *
+ * Returns: %TRUE if the sequence is empty, otherwise %FALSE.
+ *
+ * Since: 2.48
+ */
+gboolean
+g_sequence_is_empty (GSequence *seq)
+{
+  return (seq->end_node->parent == NULL) && (seq->end_node->left == NULL);
+}
+
 /**
  * g_sequence_get_end_iter:
  * @seq: a #GSequence