Do a null check on the sibling in the register allocator.

There may be a lifetime hole between the intervals, which means there is
no interval for the given position.

Litle sister of https://android-review.googlesource.com/#/c/209336/.

bug:27626705
Change-Id: I8082aa5ae2dc37d8fa5d4c430b69e6defa495439
4 files changed