We failed to clear the negative flag when handling trivial cases, so if one of the terms was 0 and the other was negative, the result would be an exact copy of the non-zero term instead of its absolute value.
If its operands were identical, cryb_mpi_add_abs() would leave the target untouched. Explicitly call mpi_zero() before returning. While there, extend the “identical operands” shortcut to also cover equality.
Both cryb_mpi_add_abs() and cryb_mpi_sub_abs() would leave the target's negative flag untouched. Explicitly clear it before returning.
