BUG/MINOR: http_htx: fix null deref in http-errors config check

BUG/MINOR: http_htx: fix null deref in http-errors config check

http-errors parsing has been refactored in a recent serie of patches.
However, a null deref was introduced by the following patch in case a
non-existent http-errors section is referenced by an "errorfiles"
directive.

  commit 2ca7601c2d6781f455cf205e4f3b52f5beb16e41
  MINOR/OPTIM: http_htx: lookup once http_errors section on check/init

Fix this by delaying ha_free() so that it is called after ha_alert().

No need to backport.