Take a lock on cs_main in DecrementNoteWitnesses

Another possibly better approach would be to take a lock inside
GetSaplingSpendDepth since we want the lock to apply to as little
code as possible and it is GetSaplingSpendDepth that calls GetDepthInMainChain
which requires a cs_main lock .

src/wallet/wallet.cpp

@@ -977,7 +977,7 @@ void CWallet::ClearNoteWitnessCache()
 
 void CWallet::DecrementNoteWitnesses(const CBlockIndex* pindex)
 {
-    LOCK(cs_wallet);
+    LOCK2(cs_main, cs_wallet);
 
     extern int32_t HUSH_REWIND;