PHP 7.2 compat

Hi.
error logs are full of this:

PHP message: PHP Warning:  A non-numeric value encountered in /wp-content/plugins/vkontakte-api/includes/comments.php on line 989
PHP message: PHP Warning:  A non-numeric value encountered in /wp-content/plugins/vkontakte-api/includes/comments.php on line 1120

get_postmeta() can return empty string or false in case there is no meta field,
it needs to be checked

something like this will suffice:

<------>public function add_tabs_button_wp() {
<------><------>global $post;
<------><------>$vkapi_comm = get_post_meta( $post->ID, 'vkapi_comm', true );
<------><------>$fbapi_comm = get_post_meta( $post->ID, 'fbapi_comm', true );
<------><------>if ($fbapi_comm != '' && $fbapi_comm != false) $fappi = (int)($fbapi_comm);
<------><------>if ($vkapi_comm != '' && $vkapi_comm != false) $vappi = (int)($vkapi_comm);
<------><------>$comm_wp    = get_comments_number() - $vappi - $fappi;
<------><------>$text       = __( 'Site', 'vkapi' );
<------><------>echo "<div>
<------><------><------>    <button style='white-space:nowrap'
<------><------><------>            class='submit'
<------><------><------>            onclick='showWP()'>
<------><------><------>        {$text} ({$comm_wp})
<------><------><------>    </button>
<------><------><------></div>";
<------>}

likewise for public function do_non_empty()