Fix invalid memory read in wxMSW wxTextCtrl::GetLineText()

Don't read before the start of the buffer when getting the text of an
empty line, this was (correctly) flagged as heap error by address
sanitizer.

src/msw/textctrl.cpp

@@ -1928,7 +1928,7 @@ wxString wxTextCtrl::GetLineText(long lineNo) const
 
         // remove the '\n' at the end, if any (this is how this function is
         // supposed to work according to the docs)
-        if ( buf[len - 1] == wxT('\n') )
+        if ( len && buf[len - 1] == wxT('\n') )
         {
             len--;
         }