Fix RBM_ZERO_AND_LOCK mode to not acquire lock on local buffers.

Commit 81c45081 introduced a new RBM_ZERO_AND_LOCK mode to ReadBuffer, which
takes a lock on the buffer before zeroing it. However, you cannot take a
lock on a local buffer, and you got a segfault instead. The version of that
patch committed to master included a check for !isLocalBuf, and therefore
didn't crash, but oddly I missed that in the back-patched versions. This
patch adds that check to the back-branches too.

RBM_ZERO_AND_LOCK mode is only used during WAL replay, and in hash indexes.
WAL replay only deals with shared buffers, so the only way to trigger the
bug is with a temporary hash index.

Reported by Artem Ignatyev, analysis by Tom Lane.

src/backend/storage/buffer/bufmgr.c

@@ -494,7 +494,8 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	 * (Note that we cannot use LockBuffer() of LockBufferForCleanup() here,
 	 * because they assert that the buffer is already valid.)
 	 */
-	if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
+	if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
+		!isLocalBuf)
 		LWLockAcquire(bufHdr->content_lock, LW_EXCLUSIVE);
 
 	if (isLocalBuf)