Avoid testing tuple visibility without buffer lock.

INSERT ... ON CONFLICT (specifically ExecCheckHeapTupleVisible) contains
another example of this unsafe coding practice.  It is much harder to get
a failure out of it than the case fixed in commit 6292c233, because in
most scenarios any hint bits that could be set would have already been set
earlier in the command.  However, Konstantin Knizhnik reported a failure
with a custom transaction manager, and it's clearly possible to get a
failure via a race condition in async-commit mode.

For lack of a reproducible example, no regression test case in this
commit.

I did some testing with Asserts added to tqual.c's functions, and can say
that running "make check-world" exposed these two bugs and no others.
The Asserts are messy enough that I've not added them to the code for now.

Report: <57EE93C8.8080504@postgrespro.ru>
Related-Discussion: <CAO3NbwOycQjt2Oqy2VW-eLTq2M5uGMyHnGm=RNga4mjqcYD7gQ@mail.gmail.com>

src/backend/executor/nodeModifyTable.c

@@ -194,6 +194,11 @@ ExecCheckHeapTupleVisible(EState *estate,
 	if (!IsolationUsesXactSnapshot())
 		return;
 
+	/*
+	 * We need buffer pin and lock to call HeapTupleSatisfiesVisibility.
+	 * Caller should be holding pin, but not lock.
+	 */
+	LockBuffer(buffer, BUFFER_LOCK_SHARE);
 	if (!HeapTupleSatisfiesVisibility(tuple, estate->es_snapshot, buffer))
 	{
 		/*
@@ -207,6 +212,7 @@ ExecCheckHeapTupleVisible(EState *estate,
 					(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 			 errmsg("could not serialize access due to concurrent update")));
 	}
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 }
 
 /*