Fix title-related backtracking with empty string

Fixes #281

Author: notriddle

lib/inlines.js

@@ -884,21 +884,20 @@ var parseReference = function(s, refmap) {
         title = this.parseLinkTitle();
     }
     if (title === null) {
-        title = "";
         // rewind before spaces
         this.pos = beforetitle;
     }
 
     // make sure we're at line end:
     var atLineEnd = true;
     if (this.match(reSpaceAtEndOfLine) === null) {
-        if (title === "") {
+        if (title === null) {
             atLineEnd = false;
         } else {
             // the potential title we found is not at the line end,
             // but it could still be a legal link reference if we
             // discard the title
-            title = "";
+            title = null;
             // rewind before spaces
             this.pos = beforetitle;
             // and instead check if the link URL is at the line end
@@ -919,7 +918,7 @@ var parseReference = function(s, refmap) {
     }
 
     if (!refmap[normlabel]) {
-        refmap[normlabel] = { destination: dest, title: title };
+        refmap[normlabel] = { destination: dest, title: title === null ? "" : title };
     }
     return this.pos - startpos;
 };

test/regression.txt

@@ -484,3 +484,11 @@ x <- 1
 <p>&amp;para</p>
 <p>¶</p>
 ````````````````````````````````
+
+#281
+```````````````````````````````` example
+[test]:example
+""third [test]
+.
+<p>&quot;&quot;third <a href="example">test</a></p>
+````````````````````````````````