Segfault for balloonGrobText on OttavaBracket

[Noeck]

Hi,

the following code produces a segmentation violation (if that's the same
as "Speicherzugriffsfehler") using 2.18.2 or 2.19.50. The combination of
\ottava and \balloonGrobText is required to cause the failure.

\version "2.19.50"
\new Staff \with { \consists "Balloon_engraver" }
\new Voice  {
  \balloonGrobText #'OttavaBracket #'(1 . 1) "A"
  \ottava #1 a4
}

The code probably worked with some 2.16.x version as I have the
corresponding pdf output. convert-ly does not change it.

Best,
Joram