BUG/MEDIUM: config: don't crash at config load time on invalid userlist names

BUG/MEDIUM: config: don't crash at config load time on invalid userlist names

Cyril Bonté reported that passing an invalid userlist name to
http_auth_group() caused haproxy to crash at load. This was due
to an attempt to use the unresolved userlist pointer later to
resolve auth groups since we report many errors before leaving
now.

This issue does not exist in earlier versions since they immediately
abort on the first error, so no backport is needed.