slashedGrace and acciaccatura missing slash for multiple notes

[Vanessa Sabino]

I could get the slash to show up for a single grace note, but the music I'm
trying to reproduce contains a group with two grace notes and in that case
the slash didn't show up. Here is an example:

\version "2.18.2"
\relative c' {
  g' ( \slashedGrace {a16 g} f4) g
}