[clang-tidy] Fix for bug 31838: readability-delete-null-pointer does not work for class members

I have made a small fix for readability-delete-null-pointer check so it also checks for class members.

Example of case that it fixes
```
  struct A {
    void foo() {
      if(mp)
        delete mp;
    }
    int *mp;
  };
```

Reviewers: JDevlieghere, aaron.ballman, alexfh, malcolm.parsons

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29726

llvm-svn: 294912