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: I0535a63ed834d64fcbf20dd8b268d109232c5077
1 file changed