BUG/MINOR: ssl/cli: fix frontend-not-found detection in 'show ssl sni -f'

BUG/MINOR: ssl/cli: fix frontend-not-found detection in 'show ssl sni -f'

Coverity (CID 1664029, GH #3466) points out that `px == NULL` after the
list_for_each_entry loop in cli_parse_show_sni() can never be true:
list_for_each_entry's loop variable, when the list is exhausted without
a break, is a container_of-derived pointer computed from the list
head's address, never NULL. This left the "-f <bogus-frontend>" case
silently falling through instead of erroring: the bug pre-dates
9993f29007, which fixed the found case (adding the missing break) but
kept comparing the wrong thing for the not-found case. Track "found"
explicitly instead.

Add reg-tests/ssl/show_ssl_sni_filter.vtc to cover the -f option,
including the not-found error case.

This must be backported to 3.2.