Fix: buffer should only be as long as the valid part of a hex string

Caught by the new node.js tests

index.js

@@ -228,7 +228,12 @@ function fromString (that, string, encoding) {
   var length = byteLength(string, encoding) | 0
   that = createBuffer(that, length)
 
-  that.write(string, encoding)
+  var actual = that.write(string, encoding)
+
+  if (actual !== length) {
+    that = that.slice(0, actual)
+  }
+
   return that
 }