Fix increment in rpc/mining.cpp when coinbase tx is skipped

Summary:
D2782 introduces an off-by-one error when fetching fees and sigopscount from pblocktemplate.

https://build.bitcoinabc.org/viewLog.html?buildId=12706&tab=buildResultsDiv&buildTypeId=BitcoinABCMasterLinux

Test Plan:

make check
test_runner.py abc-magnetic-anomaly-mining

Reviewers: deadalnix, Fabien, schancel, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2791