Fix a regression with double unlock.
Fix a regression with double unlock.

Fix a regression I introduced while refactoring
txn code with double unlock of txn->old_tuple.

We would assign txn->old_tuple before actually taking
a lock on it, then call txn_lock(), get an exception,
and in txn_rollback() unlock a lock which we didn't take.

This lead to an assertion failure or crash under concurrent
load, when the same tuple was updated from multiple connections.

mejedi/tarantool,nvoron23/tarantool,Sannis/tarantool,dkorolev/tarantool,rtsisyk/tarantool,mejedi/tarantool,ocelot-inc/tarantool,ocelot-inc/tarantool,KlonD90/tarantool,guard163/tarantool,mejedi/tarantool,Sannis/tarantool,vasilenkomike/tarantool,dkorolev/tarantool,nvoron23/tarantool,condor-the-bird/tarantool,rtsisyk/tarantool,nvoron23/tarantool,guard163/tarantool,nvoron23/tarantool,KlonD90/tarantool,dkorolev/tarantool,vasilenkomike/tarantool,Sannis/tarantool,guard163/tarantool,guard163/tarantool,vasilenkomike/tarantool,dkorolev/tarantool,KlonD90/tarantool,dkorolev/tarantool,vasilenkomike/tarantool,condor-the-bird/tarantool,ocelot-inc/tarantool,guard163/tarantool,ocelot-inc/tarantool,vasilenkomike/tarantool,condor-the-bird/tarantool,condor-the-bird/tarantool,nvoron23/tarantool,rtsisyk/tarantool,nvoron23/tarantool,mejedi/tarantool,condor-the-bird/tarantool,rtsisyk/tarantool,Sannis/tarantool,KlonD90/tarantool,KlonD90/tarantool,Sannis/tarantool