crash when unlinking an orphaned clone

[su_v]

follow-up report to bug #185734 “Crash when href of clone is invalid”
<https://bugs.launchpad.net/inkscape/+bug/185734>

Unlinking an orphaned clone (invisible object with a reference to a not-existing object) crashes inkscape.

Steps to reproduce:
1) open 'share/icons/icons.svg'
2) Ctrl+A
3) Edit > Clone > Unlink Clone
4) crash

or:

1) open 'clones_plain.svg' from bug #185734
2) Edit > Find > [x] Clones
3) Edit > Clone > Unlink Clone
4) crash

Console message:
** (inkscape:21410): CRITICAL **: SPItem* sp_use_unlink(SPUse*): assertion `orig' failed

Reproduced with Inkscape 0.47, 0.48 and 0.48+devel r9890 on OS X 10.5.8

[jazzynico (jazzynico)]

Reproduced on Windows XP, Inkscape 0.46, 0.47 and 0.48.

[Adonis Papaderos (ado-papas)]

This patch should prevent Inkscape from crashing. It does not give any visual feedback, though.

[su_v (suv-lp)]

patch tested and fix confirmed with Inkscape 0.48+devel r9899 on OS X 10.5.8:

Both test cases no longer cause Inkscape to crash when unlinking a selection which contains orphaned clones.

[jazzynico (jazzynico)]

Committed in the trunk (revision 9922) and in the 0.48.x branch (revision 9734).