fix: jobmaker crashed when consumer the first RSK message (add an initial value for previousRskWork_); use nullptr instead of NULL.
fix: jobmaker crashed when consumer the first RSK message (add an initial value for previousRskWork_); use nullptr instead of NULL.

btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool,btccom/btcpool