Revert "Fix mosh-server segfault on startup when run without arguments"

This reverts commit 578db45f

.

The NULL inputs to getaddrinfo are valid in certain cases, and at
least in the case of node, important.  The segfault happens not with
getaddrinfo, but with the error message generation below it.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>