fix a crash when reading an osgt file with multiple references to a missing texture.
fix a crash when reading an osgt file with multiple references to a missing texture.

openscenegraph/osg,jtorresfabra/osg,scrawl/osg,openscenegraph/osg,openscenegraph/osg,scrawl/osg,jtorresfabra/osg,openscenegraph/osg,jtorresfabra/osg,jtorresfabra/osg,scrawl/osg,scrawl/osg