Use optimized initializer list for CMPMetaDEx, CMPOffer

[dexX7]

CMPOffer:

• subaction was undefined

CMPMetaDex:

• still_left_for_sale is already in initializer list
• addr.empty() should be addr.clear()
• std::string has default constructor
• therefore addr.empty() was removed
• other constructor should use initializer list

Build result: https://travis-ci.org/dexX7/mastercore/builds/44918160