minor fix in Lwt.map
minor fix in Lwt.map

Ignore-this: 1ce184187e6afe31b8c2e3b99778cd7c

It was possible that [res] was connected two times, thus raising an
exception.

darcs-hash:20100528065410-c41ad-7882e8ad4ae46f0973efcc94e2cf6a49a877adcd

mirage/lwt,rgrinberg/lwt,rneswold/lwt,ocsigen/lwt,ocsigen/lwt,c-cube/lwt,fdopen/lwt,rgrinberg/lwt,fdopen/lwt,rgrinberg/lwt,mfp/lwt,rneswold/lwt,c-cube/lwt,rneswold/lwt,mirage/lwt,c-cube/lwt,gfxmonk/lwt,janestreet/lwt-async,janestreet/lwt-async,rgrinberg/lwt,mfp/lwt,gfxmonk/lwt,ocsigen/lwt