MINOR: arg: Use chunk_destroy() to release string arguments

MINOR: arg: Use chunk_destroy() to release string arguments

This way, all fields of the buffer structure are reset when a string argument
(ARGT_STR) is released.  It is also a good way to explicitly specify this kind
of argument is a chunk. So .data and .size fields must be set.

This patch may be backported to ease backports.

(cherry picked from commit 6ad7df423b71470babef545506a1af569074fc59)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 1c67ab418b2adfd99e0c9d2796af382a86ce4a0f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>