bass clef collides with accidental

[Urs Metzger]

In the following snippet the : of the bass cleff gets partly hidden by the
natural sign:

\version "2.8.1"
\score {
   << \new Staff { \clef treble a'16 a' a' a' a' a' a' a' }
      \new Staff { \clef treble c'4 \clef bass a! } >>
   \layout { ragged-right = ##t }
}