Fix capsicum-test build with GCC

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

(cherry picked from commit 47ceb65f3c213904642f4859a53a3139e9cb287d)