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.