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

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