dash: Don't crash when XML parsing fails.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 8ceecb11)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/stream_filter/dash/xml/DOMParser.cpp

@@ -65,6 +65,8 @@ bool    DOMParser::parse                    ()
         return false;
 
     this->root = this->processNode();
+    if ( this->root == NULL )
+        return false;
 
     return true;
 }