Fixed Coverity reported issue.

ID 11667: Uninitialized scalar field (UNINIT_CTOR)
Non-static class member _useVertexAttribAlias is not initialized in this constructor nor in any functions that it calls.

Member variable should have been initialized but this varaible will have always been initialized in later code
before it was used anyway so wouldn't have resulted in a runtime bug.

src/osg/ArrayDispatchers.cpp

@@ -378,6 +378,7 @@ ArrayDispatchers::ArrayDispatchers():
     _colorDispatchers(0),
     _secondaryColorDispatchers(0),
     _fogCoordDispatchers(0),
+    _useVertexAttribAlias(false),
     _useGLBeginEndAdapter(false)
 {
 
@@ -574,6 +575,7 @@ void ArrayDispatchers::reset()
 {
     if (!_initialized) init();
 
+    _useVertexAttribAlias = false;
     _useGLBeginEndAdapter = false;
 
     for(ActiveDispatchList::iterator itr = _activeDispatchList.begin();