Fixed crash when clearing up Allocator.

      * We do need a separate flag which says the allocator is used.
      * Otherwise there may be a crash when destroying the allocator,
        ... causing the nodes to be cleared and then wrongly attempt
        ... to clear children because the allocator pointer is now NULL.
	modified:   surface/include/pcl/surface/impl/poisson/octree_poisson.hpp
	modified:   surface/include/pcl/surface/impl/poisson/sparse_matrix.hpp
	modified:   surface/include/pcl/surface/poisson/octree_poisson.h
	modified:   surface/include/pcl/surface/poisson/sparse_matrix.h