Fix access to undefined index preserve-paths

We should not try to access $extra['preserve-paths'] if it is not set.

Author: tobbexiv

src/PluginWrapper.php

@@ -170,7 +170,7 @@ protected function getPreservePaths()
         $extra = $this->composer->getPackage()->getExtra();
 
         if (!isset($extra['preserve-paths'])) {
-            $paths = $extra['preserve-paths'];
+            $paths = array();
         } elseif (!is_array($extra['preserve-paths']) && !is_object($extra['preserve-paths'])) {
             $paths = array($extra['preserve-paths']);
         } else {