Fix bug with assigning a part of the string to the same string

Fix bug with assigning a part of the string to the same string

The fix has several parts:

1. don't free the old string data in ConcatSelf() if we use it as
   source
2. implement assign() using replace() rather than clear() + append()
3. fix replace() to work with replacement strings containing embedded
   NULs and optimize it by using memcpy() instead of byte-wise copy

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48302 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775