Fix UB with bench on genesis block

Summary:

During the loading of the genesis block, the bench print lines in ConnectTip divide by zero due to early return in ConnectBlock.

Increment nBlocksTotal before returning in ConnectBlock

This is a backport of Core PR15283

Test Plan: ninja && ./src/bitcoind -debug=bench | grep "Connect total:"

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8857