Segfaults involving fret diagrams

[Carl Sorensen]

David,

There have been a number of crashes involving fret diagrams when used in
lilypond book and multiple runs, for example
<http://code.google.com/p/lilypond/issues/detail?id=1997>
and
<http://code.google.com/p/lilypond/issues/detail?id=1900>

With Mike's recent discovery that non-documented property gets/sets can
lead to memory leaks, I decided to check out the properties in the fret
diagram markups.

On line 1085 of scm/fret-diagrams.scm, you added a line for the properties
of a fret-diagram-terse markup; the properties entry is
(fret-diagram-verbose-markup).

I don't understand how this properly sets the properties for a terse
diagram, and wonder if this might be responsible for the fret diagrams
crashes.  Can you shed any light on this behavior?

Thanks,

Carl