Appease the static analyzer

Since the static analyzer assumes that
`head == blockToRemove && blockToRemove->prevBlock == nullptr` may be
true, it complains that we're deleting `head` and returning `head`
shortly afterward.

Assert (without assertions, since -DNDEBUG is passed) that this isn't
the case.

Bug: 27101951
Test: mma. Warning is gone.
Change-Id: I33e98eec7b293fcf0d8826f89c287a3b870758f2