Fix an issue in PostgreSQL::Test::Cluster:psql()

Due to the commit c5385929 which made all Perl warnings to fatal, use
of PostgreSQL::Test::Cluster:psql() and safe_psql() with timeout
started to fail with the following error:

Use of uninitialized value $ret in bitwise and (&) at
..src/test/perl/PostgreSQL/Test/Cluster.pm line 2015.

Fix that by placing $ret conversion code in psql() in an if (defined
$ret) block.

With this change, the behavior of psql() becomes same as before, that
is, the whole function returns undef on timeout, which is usefully
different from returning 0.

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/06f899fd-1826-05ab-42d6-adeb1fd5e200%40eisentraut.org

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2012,12 +2012,15 @@ sub psql
 	# We don't use IPC::Run::Simple to limit dependencies.
 	#
 	# We always die on signal.
+	if (defined $ret)
+	{
 		my $core = $ret & 128 ? " (core dumped)" : "";
 		die "psql exited with signal "
 		  . ($ret & 127)
 		  . "$core: '$$stderr' while running '@psql_params'"
 		  if $ret & 127;
 		$ret = $ret >> 8;
+	}
 
 	if ($ret && $params{on_error_die})
 	{