BUG/MINOR: stick-table/cli: Check for invalid ipv6 key

BUG/MINOR: stick-table/cli: Check for invalid ipv6 key

When an ipv6 key is used to filter a CLI command on a stick table
(clear/set/show table ...), the return value of inet_pton() call must be
checked to be sure the key is valid.

This patch should fix the issue #1163. It should be backported to all
supported versions.

(cherry picked from commit b7c962b0c070a0c6ad35eaed332869a36cb931cf)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 1e850b52a1cb350620a31641452089caeb1c0318)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>