Don't cache incorrect length in wxString::assign(char*, size_t).

The length of the string here is specified in bytes and is not the same as the
string length for non-ASCII UTF-8 strings, so don't cache it as the string
length.

Just invalidate the cached length instead as we simply don't know what the
real length of the string is going to be.

See #14130.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70987 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/string.h

@@ -2669,7 +2669,7 @@ public:
 
   wxString& assign(const char *sz, size_t n)
   {
-      wxSTRING_SET_CACHED_LENGTH(n);
+      wxSTRING_INVALIDATE_CACHE();
 
       SubstrBufFromMB str(ImplStr(sz, n));
       m_impl.assign(str.data, str.len);