Prevent potential NPE

Ignore scroll events if mNestedScrollingChildRef is not set yet.
It happens if BottomSheetBehavior view is inflated from a ViewStub for
example, input events can arrive too soon.

bug: https://code.google.com/p/android/issues/detail?id=232646

Test: nullity check, existing unit tests still pass.
Change-Id: I25847ce70437d33b864b0d1b27a28d215a7c1ab3
1 file changed