trunk: crash when ungrouping a filtered group (rev >=13298)

From the comments of bug #1319837:

Steps to reproduce:
1) launch current trunk (default prefs, default new doc)
2) draw a rect
3) group it (Ctrl+G)
4) add a filter to the group (simple blur, or any preset)
5) ungroup the filtered group
--> crash:

**
ERROR:../../src/uri-references.cpp:40:Inkscape::URIReference::URIReference(SPDocument *): assertion failed: (_owner_document != NULL)

Based on tests with archived builds on OS X 10.7.5:
- not reproduced with rev <= 13297,
- reproduced with rev >= 13299;
this type of regression (crash when ungrouping a group which has a filter effect applied) was introduced with the cleanup of the style code in revision 13298:
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/13299>

Backtrace with r13397 (debug build, 64bit) on OS X 10.7.5 attached.

Test case and backtrace from FirasH:
<https://bugs.launchpad.net/inkscape/+bug/1319837/+attachment/4119023/+files/UngroupCrash.svg>
<https://bugs.launchpad.net/inkscape/+bug/1319837/+attachment/4119024/+files/UngroupCrashBacktrace>