fixed context binding issue (#33)

lib/elliptic/ec/signature.js

@@ -36,7 +36,8 @@ function getLength(buf, p) {
   }
 
   var val = 0;
-  for (var i = 0, off = p.place; i < octetLen; i++, off++) {
+  var off = p.place;
+  for (var i = 0; i < octetLen; i++, off++) {
     val <<= 8;
     val |= buf[off];
     val >>>= 0;