Fix two clang warnings

The warnings are:

```
exec.cpp:1251:55: warning: empty parentheses interpreted as a function declaration
      [-Wvexing-parse]
                const wchar_t *reader_current_filename();
                                                      ^~
exec.cpp:1251:55: note: replace parentheses with an initializer to declare a variable
                const wchar_t *reader_current_filename();
                                                      ^~
                                                       = NULL
1 warning generated.
```

the suggested fix is wrong though; and

```
expand.cpp:756:39: warning: adding 'size_t' (aka 'unsigned long') to a string does
      not append to the string [-Wstring-plus-int]
                              SELF_STR+wcslen(in+1),
                              ~~~~~~~~^~~~~~~~~~~~~
expand.cpp:756:39: note: use array indexing to silence this warning
                              SELF_STR+wcslen(in+1),
                                      ^
                              &       [            ]
expand.cpp:763:39: warning: adding 'size_t' (aka 'unsigned long') to a string does
      not append to the string [-Wstring-plus-int]
                              LAST_STR+wcslen(in+1),
                              ~~~~~~~~^~~~~~~~~~~~~
expand.cpp:763:39: note: use array indexing to silence this warning
                              LAST_STR+wcslen(in+1),
                                      ^
                              &       [            ]
2 warnings generated.
```