increaseUnlockTime uses wrong unlock time for old lock.

[lacoop6tu]

the oldLock.end should be original value but it uses the increased unlock_time

Issue in findings repo

Mitigation:
The line 513 in the VotingEscrow.sol contract :

  513      oldLocked.end = unlock_time;

Need to be replaced with the following :

  513      oldLocked.end = oldUnlockTime;