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
a07e03fd8fa7daf4d1356f7cb501ffe784ea6257.

Reported by Alexander Lakhin.

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

src/backend/access/heap/heapam.c

@@ -6229,10 +6229,9 @@ 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;

src/test/isolation/expected/intra-grant-inplace.out

@@ -63,6 +63,30 @@ step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
 step r3: ROLLBACK;
 step addk2: <... completed>
 
+starting permutation: b3 sfnku3 keyshr5 addk2 r3
+step b3: BEGIN ISOLATION LEVEL READ COMMITTED;
+step sfnku3: 
+	SELECT relhasindex FROM pg_class
+	WHERE oid = 'intra_grant_inplace'::regclass FOR NO KEY UPDATE;
+
+relhasindex
+-----------
+f          
+(1 row)
+
+step keyshr5: 
+	SELECT relhasindex FROM pg_class
+	WHERE oid = 'intra_grant_inplace'::regclass FOR KEY SHARE;
+
+relhasindex
+-----------
+f          
+(1 row)
+
+step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
+step r3: ROLLBACK;
+step addk2: <... completed>
+
 starting permutation: b2 sfnku2 addk2 c2
 step b2: BEGIN;
 step sfnku2: 

src/test/isolation/specs/intra-grant-inplace.spec

@@ -96,6 +96,14 @@ permutation
 	addk2(r3)
 	r3
 
+# reproduce bug in DoesMultiXactIdConflict() call
+permutation
+	b3
+	sfnku3
+	keyshr5
+	addk2(r3)
+	r3
+
 # same-xact rowmark
 permutation
 	b2