more optimizations for readUInt64lebn

lib/encoding/bufferreader.js

@@ -91,8 +91,16 @@ BufferReader.prototype.readUInt64BEBN = function() {
 };
 
 BufferReader.prototype.readUInt64LEBN = function() {
-  var data = Array.prototype.slice.call(this.buf, this.pos, this.pos + 8);
-  var bn = new BN(data, 10, 'le');
+  var second = this.buf.readUInt32LE(this.pos);
+  var first = this.buf.readUInt32LE(this.pos + 4);
+  var combined = (first * 0x100000000) + second;
+  var bn;
+  if (combined <= 0x1fffffffffffff) {
+    bn = new BN(combined);
+  } else {
+    var data = Array.prototype.slice.call(this.buf, this.pos, this.pos + 8);
+    bn = new BN(data, 10, 'le');
+  }
   this.pos = this.pos + 8;
   return bn;
 };