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) {
      ~~~~~~~~~~~~~~~ ^
```

RegularExpression.hxx.in

@@ -70,10 +70,10 @@ private:
  * \brief Creates an invalid match object
  */
 inline RegularExpressionMatch::RegularExpressionMatch()
+  : startp{}
+  , endp{}
+  , searchstring{}
 {
-  startp[0] = nullptr;
-  endp[0] = nullptr;
-  searchstring = nullptr;
 }
 
 /**