Fix instance of elog() called while holding a spinlock

This broke the project rule to not call any complex code while a
spinlock is held.  Issue introduced by b89e151.

Discussion: https://postgr.es/m/20200602.161518.1399689010416646074.horikyota.ntt@gmail.com
Backpatch-through: 9.5

src/backend/replication/logical/logical.c

@@ -972,6 +972,7 @@ LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart
 	{
 		slot->candidate_restart_valid = current_lsn;
 		slot->candidate_restart_lsn = restart_lsn;
+		SpinLockRelease(&slot->mutex);
 
 		elog(DEBUG1, "got new restart lsn %X/%X at %X/%X",
 			 (uint32) (restart_lsn >> 32), (uint32) restart_lsn,
@@ -979,18 +980,25 @@ LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart
 	}
 	else
 	{
+		XLogRecPtr	candidate_restart_lsn;
+		XLogRecPtr	candidate_restart_valid;
+		XLogRecPtr	confirmed_flush;
+
+		candidate_restart_lsn = slot->candidate_restart_lsn;
+		candidate_restart_valid = slot->candidate_restart_valid;
+		confirmed_flush = slot->data.confirmed_flush;
+		SpinLockRelease(&slot->mutex);
+
 		elog(DEBUG1, "failed to increase restart lsn: proposed %X/%X, after %X/%X, current candidate %X/%X, current after %X/%X, flushed up to %X/%X",
 			 (uint32) (restart_lsn >> 32), (uint32) restart_lsn,
 			 (uint32) (current_lsn >> 32), (uint32) current_lsn,
-			 (uint32) (slot->candidate_restart_lsn >> 32),
-			 (uint32) slot->candidate_restart_lsn,
-			 (uint32) (slot->candidate_restart_valid >> 32),
-			 (uint32) slot->candidate_restart_valid,
-			 (uint32) (slot->data.confirmed_flush >> 32),
-			 (uint32) slot->data.confirmed_flush
-			);
+			 (uint32) (candidate_restart_lsn >> 32),
+			 (uint32) candidate_restart_lsn,
+			 (uint32) (candidate_restart_valid >> 32),
+			 (uint32) candidate_restart_valid,
+			 (uint32) (confirmed_flush >> 32),
+			 (uint32) confirmed_flush);
 	}
-	SpinLockRelease(&slot->mutex);
 
 	/* candidates are already valid with the current flush position, apply */
 	if (updated_lsn)