calling strlen() with null pointer (#1142)

calling strlen() with null pointer (#1142)

clang analyzer found this issue.

other archive_mstring_copy_* has the pattern:

```
if (xxx == NULL) {
  aes->aes_set = 0;
  return (0);
}
```

archive_mstring_copy_utf8() didn't follow that pattern, so if NULL is passed in, it will call strlen(NULL).

Noticed that archive_mstring_copy_wcs_len() doesn't follow the pattern either.

Fixes #1142