ecore_x: ensure positive malloc size

size_ret is used later as an argument for malloc, so it should be
positive. In addition this should ensure that
ecore_x_window_porp_property_get returns a positive value and is true if
we malloc data.

Hopefully also fixes CID 1135636

src/lib/ecore_x/xlib/ecore_x_window_prop.c

@@ -545,7 +545,7 @@ ecore_x_window_prop_property_get(Ecore_X_Window win,
    if (_ecore_xlib_sync) ecore_x_sync();
    if (ret != Success)
      return 0;
-   if (!num_ret)
+   if ((!num_ret) || (size_ret <= 0))
      {
         XFree(prop_ret);
         return 0;