Return early when checking divider before child zero.

Previously the `getChildAt` method would be called with an index of -1 which
would lead to an exception being thrown and caught. This is unnecessary since
we know there will never be a divider before the first child. It also avoids
additional object creation since this method can be invoked quite frequently.

Change-Id: Iab44520d5d52f96a829a009cdd1201696edbf9a4