Check for the presence of `env` before `env.references`.

This commit fixes a "Cannot read property 'references' of undefined"
when parsing "[xxx]". state.env is sometimes undefined, and so are
references as well.

lib/rules_inline/link.js

@@ -95,7 +95,7 @@ module.exports = function link(state, silent) {
     //
     // Link reference
     //
-    if (typeof state.env.references === 'undefined') { return false; }
+    if (typeof state.env === 'undefined' || typeof state.env.references === 'undefined') { return false; }
 
     // [foo]  [bar]
     //      ^^ optional whitespace (can include newlines)