BUG/MINOR: auth: Fix a leak on error path when parsing user's groups

BUG/MINOR: auth: Fix a leak on error path when parsing user's groups

In a userlist section, when a user is parsed, if a specified group is not
found, an error is reported. In this case we must take care to release the
alredy built groups list.

It was reported by Coverity in #2841: CID 1587770.

This patch could be backported to all stable versions.

(cherry picked from commit a7f513af9187444680e3e054e0a13f818aed3307)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 8074bb26d1bbcb225100329ada2498112222e512)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>