fix BigInteger.ToByteArray() for certain negative values

[ixje]

What current issue(s) does this address, or what feature is it adding?
the audit of testnet block 678752 showed a storage deviation caused by a difference in return value for the BigInteger.ToByteArray() method for the number -48335248028225339427907476932896373492484053930

How did you solve this problem?
glanced at the C# source of the BigInteger class and applied somewhat similar logic

How did you make sure your solution works?
test cases + audit now passes for the block

Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage increased (+0.009%) to 85.113% when pulling 07095e8 on ixje:fix-bigint-tobytearray into 69a1ef5 on CityOfZion:development.