Silence compiler warnings

Rearrange a bit of code to ensure that 'mode' in LWLockRelease is
obviously always set, which seems a bit cleaner and avoids a compiler
warning (thanks to Robert for the suggestion!).

Back-patch back to 9.5 where the warning is first seen.

Author: Stephen Frost
Discussion: https://postgr.es/m/20161129152102.GR13284%40tamriel.snowman.net

Author: jconway

src/backend/storage/lmgr/lwlock.c

@@ -1781,15 +1781,14 @@ LWLockRelease(LWLock *lock)
 	 * be the latest-acquired lock; so search array backwards.
 	 */
 	for (i = num_held_lwlocks; --i >= 0;)
-	{
 		if (lock == held_lwlocks[i].lock)
-		{
-			mode = held_lwlocks[i].mode;
 			break;
-		}
-	}
+
 	if (i < 0)
 		elog(ERROR, "lock %s %d is not held", T_NAME(lock), T_ID(lock));
+
+	mode = held_lwlocks[i].mode;
+
 	num_held_lwlocks--;
 	for (; i < num_held_lwlocks; i++)
 		held_lwlocks[i] = held_lwlocks[i + 1];