Fix crash in all(?) KIPI export plugins

Scenario:
- start spectacle, take a screenshot, click *export* drop-down
  and pick *imageshack* or *remote storage* (or most of the
  others)
- spectacle crashes

Valgrind:
==60783== Invalid read of size 8
==60783==    at 0x48A149A: KIPI::PluginLoader::interface() const (invent/libkipi/src/pluginloader.cpp:385)
==60783==    by 0x14DB111A: KIPIRemoteStoragePlugin::Plugin_RemoteStorage::slotActivateExport() (invent/kipi-plugins/remotestorage/plugin_remotestorage.cpp:122)
==60783==  Address 0xc6d28b0 is 16 bytes inside a block of size 24 free'd
==60783==    at 0x484CBAC: operator delete(void*) (in /usr/local/lib/valgrind/vgpreload_memcheck-amd64-freebsd.so)
==60783==    by 0x247CAB: ExportMenu::getKipiItems() (invent/spectacle/src/Gui/ExportMenu.cpp:164)