fix: avoid crashes on "corrupted db" reindex attempts (#5717)

## Issue being fixed or feature implemented
Should fix crashes like
```
: Corrupted block database detected.
Please restart with -reindex or -reindex-chainstate to recover.
Assertion failure:
  assertion: globalInstance == nullptr
  file: mnhftx.cpp, line: 43
  function: CMNHFManager
   0#: (0x105ADA27C) stacktraces.cpp:629  - __assert_rtn
   1#: (0x104945794) mnhftx.cpp:43        - CMNHFManager::CMNHFManager(CEvoDB&)
   2#: (0x10499DA90) compressed_pair.h:40 - std::__1::__unique_if<CMNHFManager>::__unique_single std::__1::make_unique[abi:v15006]<CMNHFManager, CEvoDB&>(CEvoDB&)
   3#: (0x10499753C) init.cpp:1915        - AppInitMain(std::__1::variant<std::__1::nullopt_t, std::__1::reference_wrapper<NodeContext>, std::__1::reference_wrapper<WalletContext>, std::__1::reference_wrapper<CTxMemPool>, std::__1::reference_wrapper<ChainstateManager>, std::__1::reference_wrapper<CBlockPolicyEstimator>, std::__1::reference_wrapper<LLMQContext>> const&, NodeContext&, interfaces::BlockAndHeaderTipInfo*)
```

## What was done?


## How Has This Been Tested?

## Breaking Changes
n/a

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/init.cpp

@@ -1912,6 +1912,7 @@ bool AppInitMain(const CoreContext& context, NodeContext& node, interfaces::Bloc
                 LOCK(cs_main);
                 node.evodb.reset();
                 node.evodb = std::make_unique<CEvoDB>(nEvoDbCache, false, fReset || fReindexChainState);
+                node.mnhf_manager.reset();
                 node.mnhf_manager = std::make_unique<CMNHFManager>(*node.evodb);
 
                 chainman.Reset();