Segfaults and GC

[Joe Neeman]

The attached patch fixes a segfault/heap corruption that I was having,
but I'm not really sure why because I don't fully understand how the
guile garbage collection works.

Valgrind was complaining about reads/writes to unallocated memory from
line 57 of tuplet-number.cc. The memory had been freed in a garbage
collection sweep.

gc_segfault.patch
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