Fixed Coverity reported issue.
Fixed Coverity reported issue.

CID 11844: Uninitialized scalar field (UNINIT_CTOR)
Non-static class member _defaultValue is not initialized in this constructor nor in any functions that it calls.
Index: ../include/osgDB/Serializer



tomasthoresen/osg,marchelbling/osg,marchelbling/osg,scrawl/osg,scrawl/osg,openscenegraph/osg,marchelbling/osg,openscenegraph/osg,jtorresfabra/osg,jtorresfabra/osg,jtorresfabra/osg,artoolkit/osg,openscenegraph/osg,tomasthoresen/osg,marchelbling/osg,tomasthoresen/osg,scrawl/osg,artoolkit/osg,scrawl/osg,artoolkit/osg,artoolkit/osg,jtorresfabra/osg,openscenegraph/osg,tomasthoresen/osg