Fix '-1' problem in recv

Related with https://groups.google.com/forum/?fromgroups=#!topic/emscripten-discuss/0G8yA33ETos.

Websocket tests
```
test_websockets
test_websockets_bi
test_websockets_bi_listen
test_websockets_gethostbyname
test_websockets_bi_bigdata
test_enet
```
passed. 

But i make changes in it. I think '-1' should be tested on real projects?