Fixed regression caused by fix to issue #83. 

If a null value is found in a decimal type column then you'll get a NullReferenceException when reading it out.
Handling null values was originally fixed in pull request #78 but then it regressed for the decimal type in the fix for issue #83.

Also from what I can tell it looks like the byte ordering for decimals was working all along and the fix for issue #83 isn't needed? I verified this by writing a decimal value using the library and reading with cqlsh without any issues. Finally the tests in DecimalTypeTest.cs pass on the parent of d05b40e1a58f2a6c466c71bd51232d5272938512 (the fix for issue #83).