Fix NPE when combining pop with add transaction.

Bug 35832085

When pop transactions and an add transaction are acting
in the same frame, optimized tranactions can work properly,
but unoptimized transactions have no guarantees. This fixes
a crash with unoptimized transactions so they will at least
not break.

Test: I996f54ae2540f31134e6a9a28870826a73ea1610
Change-Id: I3890dd30e9aff70f4ac142773de8479f1a9dffc8