[clang][Interp] Avoid a dangling pointer

We've just moved all the pointers from the Block to the DeadBlock,
so make sure the old Block doesn't point to a linked list of Pointers
that don't even point to it anymore.

Author: tbaederr

clang/lib/AST/Interp/InterpBlock.cpp

@@ -106,6 +106,7 @@ DeadBlock::DeadBlock(DeadBlock *&Root, Block *Blk)
   B.Pointers = Blk->Pointers;
   for (Pointer *P = Blk->Pointers; P; P = P->Next)
     P->PointeeStorage.BS.Pointee = &B;
+  Blk->Pointers = nullptr;
 }
 
 void DeadBlock::free() {