MINOR: proxy: check if p is NULL in free_proxy()

Check if p is NULL before trying to do anything  in free_proxy(),
like most free()-like function do.

(cherry picked from commit b0281a490359d00356c81dc53f09e9aa0f5fbda5)
[cf: Backported to ease backport of a fix]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed