Fix URL rewriting error on empty URLs

Fixes GitbookIO#54, fixes GitbookIO#59

Author: Aaron O'Mullan

lib/parse/renderer.js

@@ -45,7 +45,7 @@ GitBookRenderer.prototype.link = function(href, title, text) {
 
     var o = this._extra_options;
     // Relative link, rewrite it to point to github repo
-    if(!parsed.protocol && parsed.path[0] != '/' && o && o.repo && o.dir) {
+    if(!parsed.protocol && parsed.path && parsed.path[0] != '/' && o && o.repo && o.dir) {
         href = 'https://github.com/' + o.repo + '/blob' + path.normalize(path.join(
             '/',
             o.dir,
@@ -79,7 +79,7 @@ GitBookRenderer.prototype.image = function(href, title, text) {
     var o = this._extra_options;
 
     // Relative image, rewrite it depending output
-    if(!parsed.protocol && parsed.path[0] != '/' && o && o.dir && o.outdir) {
+    if(!parsed.protocol && parsed.path && parsed.path[0] != '/' && o && o.dir && o.outdir) {
         _href = path.relative(o.outdir, path.normalize(path.join(
             o.dir,
             href