RegularExpression: Initialize RegularExpressionMatch arrays fully

The `RegularExpressionMatch` class added by commit cff58f07
(RegularExpression: New RegularExpressionMatch class, 2017-12-04) has
members with array-of-pointer types.  Use uniform initialization
syntax to initialize all array elements on construction.

This fixes a Clang scan-build 7 warning:

```
RegularExpression.hxx:138:23: warning: The left operand of '==' is a garbage value
  if (this->startp[n] == nullptr) {
      ~~~~~~~~~~~~~~~ ^
```