os: Check oc->trans_conn before using oc->fd in YieldControlNoInput

oc->trans_conn is set to NULL when the connection is closed. At this
point, oc->fd is no longer valid and shouldn't be used. Move
dereference of oc->fd up into YieldControlNoInput where the state of
oc->trans_conn can be checked in a single place.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>