Add an overload of IsUAHFenabled that that a CBlockIndex as parameter

Summary: This was it can do the null check consistently. There are at least 4 null checks when using CBlockIndex to feed this function, which is an indication that such routine is needed.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D214