Fix the NullStream class.

It is the std::streambuf that should be inherited from. The NullStream class is just a convenience class.

Without this I get the following test results:
```
The following tests FAILED:
	 21 - runSectionTests (SEGFAULT)
	 25 - runParseTOPS (SEGFAULT)
	 27 - runParseWellWithWildcards (SEGFAULT)
	 33 - runScheduleCreateFromDeck (SEGFAULT)
	 43 - runBoxTest (SEGFAULT)
	 45 - runEclipseGridCreateFromDeck (SEGFAULT)
	 46 - runEclipseStateTests (SEGFAULT)
	 48 - runScheduleTests (SEGFAULT)
	 60 - runEclipseGridTests (SEGFAULT)
Errors while running CTest
```

I am not quite sure why, it may be due implementation-defined behaviour for some ostream corner cases, anyway fixing the NullStream makes the issue go away.