Fix crash when appending items of empty vector to wxItemContainer

Passing an empty std::vector<wxString> to Append() or Insert() methods
of any wxItemContainer-derived classes, such as e.g. wxComboBox,
resulted in undefined behaviour due to accessing the first element of an
empty vector.

Fix this by avoiding using it when the vector is empty.

include/wx/arrstr.h

@@ -494,7 +494,7 @@ public:
     wxArrayStringsAdapter(const std::vector<wxString>& strings)
         : m_type(wxSTRING_POINTER), m_size(strings.size())
     {
-        m_data.ptr = &strings[0];
+        m_data.ptr = m_size == 0 ? NULL : &strings[0];
     }
 #endif // wxUSE_STD_CONTAINERS_COMPATIBLY