Fix performance of rcube_db_mysql::get_variable()

As currently we're using this to find only max_allowed_packet value,
it is better to use "SHOW VARIABLES LIKE ?" instead of asking for all variables.

CHANGELOG

@@ -9,6 +9,7 @@ CHANGELOG Roundcube Webmail
 - Fix setting max packet size for DB caches and check packet size also in shared cache
 - Fix needless security warning on BMP attachments display (#1490282)
 - Fix handling of some improper constructs in format=flowed text as per the RFC3676[4.5] (#1490284)
+- Fix performance of rcube_db_mysql::get_variable()
 
 RELEASE 1.1.0
 -------------

program/lib/Roundcube/rcube_db_mysql.php

@@ -161,15 +161,24 @@ class rcube_db_mysql extends rcube_db
     {
         if (!isset($this->variables)) {
             $this->variables = array();
+        }
+
+        if (array_key_exists($varname, $this->variables)) {
+            return $this->variables[$varname];
+        }
 
-            $result = $this->query('SHOW VARIABLES');
+        $result = $this->query('SHOW VARIABLES LIKE ?', $varname);
 
         while ($row = $this->fetch_array($result)) {
             $this->variables[$row[0]] = $row[1];
         }
+
+        // not found, use default
+        if (!isset($this->variables[$varname])) {
+            $this->variables[$varname] = $default;
         }
 
-        return isset($this->variables[$varname]) ? $this->variables[$varname] : $default;
+        return $this->variables[$varname];
     }
 
     /**