src: silence compiler warning node_process_methods

Currently, the following compiler warning is generated by clang:

../src/node_process_methods.cc:71:3:
warning: indirection of non-volatile null pointer will be deleted,
not trap [-Wnull-dereference]
  *static_cast<volatile void**>(nullptr) = nullptr;
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/node_process_methods.cc:71:3: note:
consider using __builtin_trap() or qualifying pointer with 'volatile'
1 warning generated.

This commit adds the volatile qualifier to avoid this warning.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines