Potential deadlock in reorg() due to unreleased txLookupLock on error paths

[userInner]

In core/blockchain.go, the reorg function acquires a write lock via bc.txLookupLock.Lock(). However, several early return paths (specifically when bc.GetBlock returns nil) fail to call bc.txLookupLock.Unlock().

Lines leading to leak:

[Line 2651]: return errInvalidOldChain
[Line 2670]: return errInvalidOldChain
[Line 2685]: return errInvalidNewChain

This will cause any subsequent transaction lookups or further reorgs to hang indefinitely.

Proposed Fix:
Use defer bc.txLookupLock.Unlock() right after bc.txLookupLock.Lock() and remove the manual Unlock() at the end of the function to ensure the lock is always released.

[userInner]

I can provide a PR to fix this using defer. Would that be welcome?