[Container.Grid] SparseGridTopology: Fix dangling reference

[fredroy]

Using the reference from top() after a pop() is Undefined Behavior (dangling reference)

Detected with AddressSanitizer

==41501==ERROR: AddressSanitizer: container-overflow on address 0x621001a70d00 at pc 0x00010d3f1730 bp 0x00016d0ee700 sp 0x00016d0ee6f8
READ of size 4 at 0x621001a70d00 thread T0
    #0 0x00010d3f172c in sofa::component::topology::container::grid::SparseGridTopology::propagateFrom(sofa::type::Vec<3u, int> const&, boost::intrusive_ptr<sofa::component::topology::container::grid::RegularGridTopology>, sofa::type::vector<sofa::component::topology::container::grid::SparseGridTopology::Type, sofa::type::CPUMemoryManager<sofa::component::topology::container::grid::SparseGridTopology::Type>>&, sofa::type::vector<bool, sofa::type::CPUMemoryManager<bool>>&, std::__1::stack<sofa::type::Vec<3u, int>, std::__1::deque<sofa::type::Vec<3u, int>, std::__1::allocator<sofa::type::Vec<3u, int>>>>&) const SparseGridTopology.cpp:1440
    #1 0x00010d3f0b70 in sofa::component::topology::container::grid::SparseGridTopology::launchPropagationFromSeed(sofa::type::Vec<3u, int> const&, boost::intrusive_ptr<sofa::component::topology::container::grid::RegularGridTopology>, sofa::type::vector<sofa::component::topology::container::grid::SparseGridTopology::Type, sofa::type::CPUMemoryManager<sofa::component::topology::container::grid::SparseGridTopology::Type>>&, sofa::type::vector<bool, sofa::type::CPUMemoryManager<bool>>&, std::__1::stack<sofa::type::Vec<3u, int>, std::__1::deque<sofa::type::Vec<3u, int>, std::__1::allocator<sofa::type::Vec<3u, int>>>>&) const SparseGridTopology.cpp:890
.......

[with-all-tests]

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