x11-toolkits/open-motif*: MenuShell: computing position uses RC_Type() expecting RowColumn

To compute the position of a menu to display, in some cases the
position is computed invoking RC_Type() that expects a RowColumn
widget. One case was not verifying this hence leading to coredump.

Obtained from: https://sourceforge.net/p/motif/code/merge-requests/5/