Handle the case where no chain tip is available, in InvalidChainFound(). This fixes a null pointer deference when running unit tests, if the genesis block or block validation code is broken.
Split GetNextWorkRequired() into two functions to allow the difficulty calculations to
be tested without requiring a full blockchain.
Add unit tests to cover basic difficulty calculation, plus each of the min/max actual
time, and maximum difficulty target conditions.
This corrects a bug where an exception thrown reading from the database causes the cursor to
be left open, which causes an assertion error to occur when the database is deleted (around
line 938 of init.cpp).
Ross Nicoll