Clear pindexBestHeader in ChainstateManager::Unload()

-----

Code Reviewer Notes

Call graph of relevant functions:

UnloadBlockIndex() <-- Moved from
    calls ChainstateManager::Unload() <-- Moved to

Safe because ChainstateManager::Unload() is called only by
UnloadBlockIndex() and no other callers.

src/validation.cpp

@@ -4122,7 +4122,6 @@ void UnloadBlockIndex(CTxMemPool* mempool, ChainstateManager& chainman)
 {
     AssertLockHeld(::cs_main);
     chainman.Unload();
-    chainman.pindexBestHeader = nullptr;
     if (mempool) mempool->clear();
     g_versionbitscache.Clear();
     for (int b = 0; b < VERSIONBITS_NUM_BITS; b++) {
@@ -5171,6 +5170,7 @@ void ChainstateManager::Unload()
 
     m_failed_blocks.clear();
     m_blockman.Unload();
+    pindexBestHeader = nullptr;
     m_best_invalid = nullptr;
 }