PG-1931 Fix returned vaules from C function

Latest version of extension stll may be installed with previous API
verison. So we have to return appropriate data set from C code. This
commit fixes bug that was introduced in PGSM version 2.1 when extension
created with API version 2.0 crashes on data fetch.

t/034_update.pl

@@ -0,0 +1,54 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use File::Basename;
+use File::Compare;
+use Test::More;
+use lib 't';
+use pgsm;
+
+# Get filename and create out file name and dirs where requried
+PGSM::setup_files_dir(basename($0));
+
+# Create new PostgreSQL node and do initdb
+my $node = PGSM->pgsm_init_pg();
+my $pgdata = $node->data_dir;
+
+# Update postgresql.conf to include/load pg_stat_monitor library
+$node->append_conf('postgresql.conf', "shared_preload_libraries = 'pg_stat_monitor'");
+
+# Start server
+my $rt_value = $node->start;
+ok($rt_value == 1, "Start Server");
+
+# Create EXTENSION version 2.0
+my ($cmdret, $stdout, $stderr) = $node->psql('postgres', 'CREATE EXTENSION pg_stat_monitor VERSION "2.0";', extra_params => ['-a']);
+ok($cmdret == 0, "Create PGSM EXTENSION");
+PGSM::append_to_debug_file($stdout);
+
+# Check that we have some results for version 2.0
+($cmdret, $stdout, $stderr) = $node->psql('postgres', "SELECT * FROM pg_stat_monitor;", extra_params => ['-a']);
+ok($cmdret == 0, "Check PGSM returns some results for version 2.0");
+PGSM::append_to_debug_file($stdout);
+
+# Update EXTENSION to new version 2.1
+($cmdret, $stdout, $stderr) = $node->psql('postgres', 'ALTER EXTENSION pg_stat_monitor UPDATE TO "2.1";', extra_params => ['-a']);
+ok($cmdret == 0, "Update PGSM EXTENSION to new version");
+PGSM::append_to_debug_file($stdout);
+
+# Check that we have some results for version 2.1
+($cmdret, $stdout, $stderr) = $node->psql('postgres', "SELECT * FROM pg_stat_monitor;", extra_params => ['-a']);
+ok($cmdret == 0, "Check PGSM returns some results for version 2.1");
+PGSM::append_to_debug_file($stdout);
+
+# DROP EXTENSION
+$stdout = $node->safe_psql('postgres', 'DROP EXTENSION pg_stat_monitor;', extra_params => ['-a']);
+ok($cmdret == 0, "DROP PGSM EXTENSION");
+PGSM::append_to_debug_file($stdout);
+
+# Stop the server
+$node->stop;
+
+# Done testing for this testcase file.
+done_testing();