DNP3 Protocol Bugfix / Enhancement

- Fix for Single-bit Objects offset-pointer
- Enhancement for Obj 12 Var 2 (Pattern Control Block) and Var 3 (Pattern Mask) Decoding

Bug: 10558
Change-Id: I8d3f6cee4acbab09d0b93dab6b868cddd842b682
Reviewed-on: https://code.wireshark.org/review/4597
Reviewed-by: Michael Mann <mmann78@netscape.net>