Fix bug in socket write optimization

Thanks to tlb@tlb.org for finding it!

lib/net.js

@@ -624,7 +624,12 @@ Stream.prototype._writeOut = function (data, encoding) {
 
   if (bytesWritten == len) {
     // awesome. sent to buffer.
-    buffer.used -= len; // Optimization - save the space
+    if (buffer === pool) {
+      // If we're just writing from the pool then we can make a little
+      // optimization and save the space.
+      buffer.used -= len;
+    }
+
     if (queuedData) {
       return false;
     } else {