Fix caching wrong length in wxString(str, len) ctor in UTF-8 build.

A length greater than that of the source string could be passed to this ctor.
This worked correctly, i.e. created a string which was a copy of the source
one but cached a wrong length for it.

Avoid this by explicitly checking the length before caching it in
wxString::assign(str, len).

See #14130.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/strings/strings.cpp

@@ -190,6 +190,8 @@ void StringTestCase::Constructors()
     CPPUNIT_ASSERT_EQUAL( L"Hello", wxString(L"Hello", 5) );
 #endif // wxUSE_UNICODE
 
+    CPPUNIT_ASSERT_EQUAL( 0, wxString(wxString(), 17).length() );
+
     static const char *s = "?really!";
     const char *start = wxStrchr(s, 'r');
     const char *end = wxStrchr(s, '!');