http: correct fencepost error when freeing sockets

This only becomes apparent when using a small maxSockets for pipelining
requests on a keep-alive connection. Bug introduced in 65f6f06a.

This change doesn't break `make test jslint`, which indicates that there need to be some more tests written around this stuff. Will get to that when I get a chance.

This only affects master, as the keep-alive behavior is very different in 0.10 and earlier.