Fix compiler warnings

Fixes the following compiler warning

```
miner.cpp: In constructor ‘COrphan::COrphan(const CTransaction*)’:
miner.cpp:69:14: warning: ‘COrphan::feeRate’ will be initialized after [-Wreorder]
     CFeeRate feeRate;
              ^
miner.cpp:68:12: warning:   ‘double COrphan::dPriority’ [-Wreorder]
     double dPriority;
            ^
miner.cpp:71:5: warning:   when initialized here [-Wreorder]
     COrphan(const CTransaction* ptxIn) : ptx(ptxIn), feeRate(0), dPriority(0)
```