fix link references that are reserved words, like [constructor]()

Author: hegemonic

lib/markdown.js

@@ -1512,7 +1512,7 @@ function convert_tree_to_html( tree, references, options ) {
       jsonml[ 0 ] = "a";
 
       // grab this ref and clean up the attribute node
-      var ref = references[ attrs.ref ];
+      var ref = Object.prototype.hasOwnProperty.call(references, attrs.ref) && references[ attrs.ref ];
 
       // if the reference exists, make the link
       if ( ref ) {
@@ -1536,7 +1536,7 @@ function convert_tree_to_html( tree, references, options ) {
       jsonml[ 0 ] = "img";
 
       // grab this ref and clean up the attribute node
-      var ref = references[ attrs.ref ];
+      var ref = Object.prototype.hasOwnProperty.call(references, attrs.ref) && references[ attrs.ref ];
 
       // if the reference exists, make the link
       if ( ref ) {