auth-juniper.c: ignore non-empty lines from TNCC after DSPREAUTH cookie

auth-juniper.c: ignore non-empty lines from TNCC after DSPREAUTH cookie

This skips over a seemingly harmless DSPREAUTH failure:

   Unexpected non-empty line from TNCC after DSPREAUTH cookie: '0'
   Failed to read response from TNCC
   Failed to obtain WebVPN cookie

After the unexpected '0', TNCC sends an empty line response and the
authentication sequence can proceed normally. In case other TNCC
variants send more chatter, the function ignores and logs up to 10
non-empty lines before giving up.

Signed-off-by: Marc St-Amand <pedalling.philosopher@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>