util: Add Assert identity function

The utility is primarily useful to dereference pointer types, which are
known to be not null at that time.

For example, the ArgsManager is known to exist when the wallets are
started. Instead of silently relying on that assumption, Assert can be
used to abort the program and avoid UB should the assumption ever be
violated.

src/test/util/mining.cpp

@@ -11,6 +11,7 @@
 #include <node/context.h>
 #include <pow.h>
 #include <script/standard.h>
+#include <util/check.h>
 #include <validation.h>
 
 CTxIn generatetoaddress(const NodeContext& node, const std::string& address)
@@ -39,9 +40,8 @@ CTxIn MineBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
 
 std::shared_ptr<CBlock> PrepareBlock(const NodeContext& node, const CScript& coinbase_scriptPubKey)
 {
-    assert(node.mempool);
     auto block = std::make_shared<CBlock>(
-        BlockAssembler{*node.mempool, Params()}
+        BlockAssembler{*Assert(node.mempool), Params()}
             .CreateNewBlock(coinbase_scriptPubKey)
             ->block);