[libunity-2d] Fix potential crash because of access to freed memory.

See this Valgrind output when running valgrind unity-2d-launcher:

==8338== Invalid read of size 1
==8338== at 0x82CD0F6: ??? (in /usr/lib/libQtCore.so.4.7.2)
==8338== by 0x82D388D: QObject::connect(QObject const*, char const*, QObject const*, char const*, Qt::ConnectionType) (in /usr/lib/libQtCore.so.4.7.2)
==8338== by 0x40F461: QObject::connect(QObject const*, char const*, char const*, Qt::ConnectionType) const (qobject.h:315)
==8338== by 0x406CCEC: PropertyBinder::bind(QObject*, char const*, QObject*, char const*) (propertybinder.cpp:55)
==8338== by 0x40E1D2: main (launcher.cpp:121)
==8338== Address 0x2a681cb8 is 24 bytes inside a block of size 47 free'd
==8338== at 0x4C282ED: free (vg_replace_malloc.c:366)
==8338== by 0x40F352: QByteArray::~QByteArray() (in /home/aurelien/opt/kde4-ubuntu/bin/unity-2d-launcher)
==8338== by 0x406CB38: signalNameFromMethod(QMetaMethod*) (propertybinder.cpp:37)
==8338== by 0x406CCD0: PropertyBinder::bind(QObject*, char const*, QObject*, char const*) (propertybinder.cpp:55)
==8338== by 0x40E1D2: main (launcher.cpp:121)