Stop reading uninitialized memory in heap_inplace_lock().

Stop computing a never-used value.  This removes the read; the read had
no functional implications.  Back-patch to v12, like commit
a07e03fd8f.

Reported by Alexander Lakhin.

Discussion: https://postgr.es/m/6c92f59b-f5bc-e58c-9bdd-d1f21c17c786@gmail.com

src/backend/access/heap/heapam.c

@@ -6118,10 +6118,9 @@ heap_inplace_lock(Relation relation,
 			LockTupleMode lockmode = LockTupleNoKeyExclusive;
 			MultiXactStatus mxact_status = MultiXactStatusNoKeyUpdate;
 			int			remain;
-			bool		current_is_member;
 
 			if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
-										lockmode, &current_is_member))
+										lockmode, NULL))
 			{
 				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 				ret = false;