Fix dereferencing invalid iterator in wxString in UTF-8 build

Fix iterator going past end of string in PosLenToImpl, it can't become
end() as the end iterator can't be dereferenced.

See #23305.

(cherry picked from commit 73ad17db4ec9a48e64ed40f5020918ec9a05a8a5)

docs/changes.txt

@@ -240,6 +240,7 @@ All:
 - Add move ctor and assignment to wxString (Pavel Tyunin, #23224).
 - Enable large file support in Unix CMake builds (Maarten Bent, #22750).
 - Update Georgian translations (NorwayFun, #22673).
+- Don't use invalid iterator in wxString in UTF-8 build (Ian Day, #23305).
 
 All (GUI):
 

src/common/string.cpp

@@ -262,7 +262,7 @@ void wxString::PosLenToImpl(size_t pos, size_t len,
             // going beyond the end of the string, just as std::string does
             const const_iterator e(end());
             const_iterator i(b);
-            while ( len && i <= e )
+            while ( len && i < e )
             {
                 ++i;
                 --len;