Fix error when user-configured skin does not exist anymore (#7271)

We fallback to the system skin not the default one.

CHANGELOG

@@ -10,6 +10,7 @@ CHANGELOG Roundcube Webmail
 - Mailvelope: Use sender's address to find pubkeys to check signatures (#7348)
 - Mailvelope: Fix Encrypt button hidden in Elastic (#7353)
 - Fix PHP warning: count(): Parameter must be an array or an object... in ID command handler (#7392)
+- Fix error when user-configured skin does not exist anymore (#7271)
 
 RELEASE 1.4.4
 -------------

program/include/rcmail_output_html.php

@@ -275,7 +275,9 @@ EOF;
     public function set_skin($skin)
     {
         if (!$this->check_skin($skin)) {
-            $skin = rcube_config::DEFAULT_SKIN;
+            // If the skin does not exist (could be removed or invalid),
+            // fallback to the skin set in the system configuration (#7271)
+            $skin = $this->config->system_skin;
         }
 
         $skin_path = 'skins/' . $skin;

program/lib/Roundcube/rcube_config.php

@@ -27,6 +27,8 @@ class rcube_config
 {
     const DEFAULT_SKIN = 'elastic';
 
+    public $system_skin = 'elastic';
+
     private $env       = '';
     private $paths     = array();
     private $prop      = array();
@@ -231,6 +233,8 @@ class rcube_config
             $this->prop['skin'] = self::DEFAULT_SKIN;
         }
 
+        $this->system_skin = $this->prop['skin'];
+
         // fix paths
         foreach (array('log_dir' => 'logs', 'temp_dir' => 'temp') as $key => $dir) {
             foreach (array($this->prop[$key], '../' . $this->prop[$key], RCUBE_INSTALL_PATH . $dir) as $path) {
@@ -452,7 +456,7 @@ class rcube_config
         }
 
         if ($prefs['skin'] == 'default') {
-            $prefs['skin'] = self::DEFAULT_SKIN;
+            $prefs['skin'] = $this->system_skin;
         }
 
         $skins_allowed = $this->get('skins_allowed');