crosh: network_diag: validate hostnames/IPs

If you typo an address to probe, you get a stream of resolution errors
if it's an invalid DNS name.  Do some sanity checking up front so the
user knows what's going on vs getting even more confused.

BUG=chromium:766271
TEST=`network_diag 127.0.0.1;` throws a nice error now

Change-Id: Iee02f197748187441de452dd806bba991d945fcf
(cherry picked from commit e759936a8922d70969670fa3dba01e132be03994)
Reviewed-on: https://chromium-review.googlesource.com/673923
Reviewed-by: Brian Norris <briannorris@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed