Fix capsicum-test build with GCC

Apparently GCC defines NULL to 0 in C++11 mode (instead of nullptr), so
this causes the following error:
```
In file included from capsicum-test.h:15,
from capsicum-test.cc:1:
gtest-1.10.0/include/gtest/gtest.h: In instantiation of 'testing::AssertionResult testing::internal::CmpHelperNE(const char*, const char*, const T1&, const T2&) [with T1 = long int; T2 = procstat*]':
capsicum-test.cc:75:3: required from here
gtest-1.10.0/include/gtest/gtest.h:1621:28: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
1609 | if (val1 op val2) {\
| ~~~~~~~~~~~~
......
1621 | GTEST_IMPL_CMP_HELPER_(NE, !=);
gtest-1.10.0/include/gtest/gtest.h:1609:12: note: in definition of macro 'GTEST_IMPL_CMP_HELPER_'
1609 | if (val1 op val2) {\
| ^~
```

Fix this by using nullptr directly.

Submitted upstream as https://github.com/google/capsicum-test/pull/56

Reported by: Jenkins CI