Fixes #1551, Prevent releasing the exclusive lock twice on rebuild.

[bmallin]

Since mustExit is never set to false, the finally block will always attempt to release the lock. This is fixed by simply setting mustExit to false if we release the lock before executing the finally block.

[JensSchadron]

Hi @bmallin, thank you for your contribution. It's very much appreciated.
However, sadly, we'll have to decline it as the code in which the fix has been made has been changed once more as part of a rework of the transaction system. See PR #1560 🙂 .