cnat: fix cnat_set_snat() debug CLI

Otherwise, the debug CLI command is unusable in a script because it
will eat (and complain about) subsequent lines in the script. Missing
this guitar lick, etc:

/* Get a line of input. */
 if (!unformat_user (input, unformat_line_input, line_input))
   return 0;

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Id328e6f1cc4d2e1672c3946db3865ab5a1a3af8d