Fixed Coverity reported issue.

CID 11690: Uninitialized scalar field (UNINIT_CTOR)
Non-static class member _frameLastUsed is not initialized in this constructor nor in any functions that it calls.

src/osg/BufferObject.cpp

@@ -49,6 +49,7 @@ GLBufferObject::GLBufferObject(unsigned int contextID, BufferObject* bufferObjec
     _set(0),
     _previous(0),
     _next(0),
+    _frameLastUsed(0),
     _extensions(0)
 {
     assign(bufferObject);