Fix leak of anonymous OnCancelListener reference

Author: AmandaRiu

vendored/WordPressLoginFlow/src/main/java/org/wordpress/android/login/LoginBaseFormFragment.java

@@ -237,6 +237,7 @@ protected void endProgress() {
 
         if (mProgressDialog != null) {
             mProgressDialog.cancel();
+            mProgressDialog.setOnCancelListener(null);
             mProgressDialog = null;
         }