Cancel CV sleep during subtransaction abort.

Generally, error recovery paths that need to do things like
LWLockReleaseAll and pgstat_report_wait_end also need to call
ConditionVariableCancelSleep, but AbortSubTransaction was missed.

Since subtransaction abort might destroy up the DSM segment that
contains the ConditionVariable stored in cv_sleep_target, this
can result in a crash for anything using condition variables.

Reported and diagnosed by Andres Freund.

Discussion: http://postgr.es/m/20171221110048.rxk6464azzl5t2fi@alap3.anarazel.de

src/backend/access/transam/xact.c

@@ -4596,6 +4596,9 @@ AbortSubTransaction(void)
 	/* Reset WAL record construction state */
 	XLogResetInsertion();
 
+	/* Cancel condition variable sleep */
+	ConditionVariableCancelSleep();
+
 	/*
 	 * Also clean up any open wait for lock, since the lock manager will choke
 	 * if we try to wait for another lock before doing this.