Fix perl warning from commit 9b4eafcaf4

per gripe from Andres Freund and Tom Lane

Backpatch to all live branches.

src/test/perl/PostgresNode.pm

@@ -1519,7 +1519,7 @@ sub _reserve_port
 	# take an exclusive lock to avoid concurrent access
 	flock($portfile, LOCK_EX) || die "locking port file $filename: $!";
 	# see if someone else has or had a reservation of this port
-	my $pid = <$portfile>;
+	my $pid = <$portfile> || "0";
 	chomp $pid;
 	if ($pid +0 > 0)
 	{