Fix missing call to processReadyForQuery

There was still one place which didn't update the transaction status
indicator based on the ReadyForQuery message.  Prior to this fix, a call
to Commit might have sometimes erroneously complained about the command
tag being ROLLBACK instead of returning the correct error, "Could not
complete operation in a failed transaction".  Though such an application
would have had to ignore a failed query in the first place, which would
explain the lack of bug reports.

Found while investigating a report from Eric Chlebek in #225, though I
can't come up with a scenario where his problem would have been caused
by this.