Fix 2 previous isolated high-surrogate code unit handling (#300)

Closes #299.

Author: tats-u

lib/inlines.js

@@ -320,7 +320,7 @@ var scanDelims = function(cc) {
         // NaN & 0xfc00 = 0
         // checks if 2 previous char is high surrogate
         if ((two_previous_cc & 0xfc00) !== 0xd800) {
-            return previous_char;
+            return str.charAt(pos - 1);
         }
         return str.slice(pos - 2, pos);
     }