Make gl_pi_cr exception-safe

It used to be the case that we could be interrupted or run out of
memory after pushing nulls onto the two arrays capturing past
computations, but before changing the nulls to real values. This
left us in a corrupted state.

Bug: 117705512
Test: crcalc tests, Calculator tests
Change-Id: I35a5cf97967ede6fa298c178b5056461906bea1e