Fix fatal error "Call to a member function isMuPlugin() on null"

In certain configurations, PUC could call a method that uses the $package property before calling the parent class constructor that initialises that property. This has now been fixed by moving the parent constructor call a few lines up.

Fixes #335

Puc/v4p8/Plugin/UpdateChecker.php

@@ -56,6 +56,8 @@ public function __construct($metadataUrl, $pluginFile, $slug = '', $checkPeriod
 			}
 			add_filter($slugCheckFilter, array($this, 'getAbsolutePath'));
 
+			parent::__construct($metadataUrl, dirname($this->pluginFile), $slug, $checkPeriod, $optionName);
+
 			//Backwards compatibility: If the plugin is a mu-plugin but no $muPluginFile is specified, assume
 			//it's the same as $pluginFile given that it's not in a subdirectory (WP only looks in the base dir).
 			if ( (strpbrk($this->pluginFile, '/\\') === false) && $this->isUnknownMuPlugin() ) {
@@ -66,8 +68,6 @@ public function __construct($metadataUrl, $pluginFile, $slug = '', $checkPeriod
 			//Details: https://github.com/YahnisElsts/plugin-update-checker/issues/138#issuecomment-335590964
 			add_action('uninstall_' . $this->pluginFile, array($this, 'removeHooks'));
 
-			parent::__construct($metadataUrl, dirname($this->pluginFile), $slug, $checkPeriod, $optionName);
-
 			$this->extraUi = new Puc_v4p8_Plugin_Ui($this);
 		}