clang fix

At 42 (OS X 10.9.5)

> cc -v
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.0.0
Thread model: posix

On my laptop (OS X 10.10.1, latest dev tools)

> cc -v
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix

If I try to run the get_next_line tests, I run into ```error: expression which evaluates to zero treated as a null pointer constant of type 'char *'```.

This simple fix allows the tests to compile on both versions, with identical results.