Correctly emulate pre-fork-4 behavior to allow us to safely validate old blocks without asserting or accessing uninitialized memory.  Fixes some crashes reported, like one reported in BitShares/bitshares_toolkit#647.  This is based on a previous fix which was un-merged when market_engine.cpp was moved to market_engine_v2.cpp around the time of 0.4.13
Merge pull request #55 from emfrias/emulate_pre_fork4_behavior

Correctly emulate pre-fork-4 behavior to allow us to safely validate old...