Fix crash due to other threads using the model at the time it gets deleted. No proper solution found though, with the current API.

Running nepomukqueryservice in valgrind gave this:

"/d/kde/inst/kde4.9/bin/nepomukservicestub(19732)" Soprano: "Timeout (5)": "Command timed out."
==19732== Thread 3:
==19732== Invalid read of size 8
==19732==    at 0xC5AB6DA: QPointer<Soprano::Client::ClientConnection>::operator->() const (qpointer.h:74)
==19732==    by 0xC5AA70A: Soprano::Client::ClientModel::executeQuery(QString const&, Soprano::Query::QueryLanguage, QString const&) const (clientmodel.cpp:106)
==19732==    by 0x68E9319: Nepomuk2::MainModel::executeQuery(QString const&, Soprano::Query::QueryLanguage, QString const&) const (nepomukmainmodel.cpp:182)
==19732==    by 0x12BDDEED: Nepomuk2::Query::SearchRunnable::run() (searchrunnable.cpp:89)
==19732==    by 0x4ECA2FC: QThreadPoolThread::run() (qthreadpool.cpp:107)
==19732==    by 0x4EDAF27: QThreadPrivate::start(void*) (qthread_unix.cpp:307)
==19732==    by 0x53CCF04: start_thread (in /lib64/libpthread-2.14.1.so)
==19732==    by 0x86CD10C: clone (in /lib64/libc-2.14.1.so)
==19732==  Address 0x117af938 is 72 bytes inside a block of size 80 free'd
==19732==    at 0x4C28706: operator delete(void*) (vg_replace_malloc.c:457)
==19732==    by 0xC5AA2ED: Soprano::Client::ClientModel::~ClientModel() (clientmodel.cpp:58)
==19732==    by 0x68E8BAB: (anonymous namespace)::GlobalModelContainer::init(bool) (nepomukmainmodel.cpp:82)
==19732==    by 0x68E8E58: (anonymous namespace)::GlobalModelContainer::model() (nepomukmainmodel.cpp:100)
==19732==    by 0x68E932D: Nepomuk2::MainModel::executeQuery(QString const&, Soprano::Query::QueryLanguage, QString const&) const (nepomukmainmodel.cpp:183)
==19732==    by 0x12BDDEED: Nepomuk2::Query::SearchRunnable::run() (searchrunnable.cpp:89)
==19732==    by 0x4ECA2FC: QThreadPoolThread::run() (qthreadpool.cpp:107)
==19732==    by 0x4EDAF27: QThreadPrivate::start(void*) (qthread_unix.cpp:307)
==19732==    by 0x53CCF04: start_thread (in /lib64/libpthread-2.14.1.so)
==19732==    by 0x86CD10C: clone (in /lib64/libc-2.14.1.so)
==19732== 

No crash anymore after this "fix"...