ViewRootImpl: Guard against null in performMeasure

In particular we are seeing this in the call sites from performTraversals
in monkey crashes. I don't have exact repro but it seems like a feasible
state to get in to...for example...WindowManagerGlobal#addView can trigger
removal of a dying view immediately without respect for the mIsInTraversal
flag when it calls doDie(). This means we can dispatch detached from window
setting mView == null while performing a traversal. There's some question
about why this doDie is even required but...seems a little nerve wrecking
to change at the moment and it seems best to just guard against null for now.

Test: Monkeys will test.
Bug: 37343098
Change-Id: I94f2569c1ef70819c083f2b2b34b59622e6c6260