when adding \new Staff to a StaffGroup, ledger lines not rendered if clef is specified.

[Andy Deitrich]

% The example here renders perfectly if \clef bass is removed
% but as is, the ledger lines in the new staff are not rendered.

\version "2.18.2"

\new StaffGroup \relative c {
        \time 6/8
        \new Staff
        c''2. | c2. 
        <<
        {<bes des f>2.~ | <bes des f>}
        \new Staff 
        \clef bass 
        {ges,,2. | f}
        >>
        }