Add UB check to FreeListAllocator::deallocate
#2658
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This, together with the existing check that the suballocation is not already free, ensures that all UB that can arise from
FreeListAllocator::deallocateis caught with debug assertions on. Of course, since there are raw pointers in use, it's always going to stay unsafe because the user could supply a pointer with the right address but the wrong provenance. But that would require going out of your way to do that specifically. On the other hand, using something "safe" like an index means that it might not be possible to catch the UB where you allocate using one suballocator and deallocate using another because indices are relative, which this catches because pointers are absolute. So in some sense I consider this safer.