Avoid dereferencing invalid iterator in wxMessageDialog code.

The iterator wxString::rbegin().base()+1 is invalid so check that we don't use
it.

Closes #13126.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67407 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/msgdlg.cpp

@@ -253,8 +253,10 @@ void wxMessageDialog::ReplaceStaticWithEdit()
     {
         if ( *i != '\n' )
         {
-            // found last non-newline char, remove everything after it and stop
-            text.erase(i.base() + 1, text.end());
+            // found last non-newline char, remove anything after it if
+            // necessary and stop in any case
+            if ( i != text.rbegin() )
+                text.erase(i.base() + 1, text.end());
             break;
         }
     }