BUG/MINOR: proxy: Missing calloc return value check in proxy_defproxy_cpy

BUG/MINOR: proxy: Missing calloc return value check in proxy_defproxy_cpy

A memory allocation failure happening in proxy_defproxy_cpy while
copying the default compression options would have resulted in a crash.
This function is called for every new proxy found while parsing the
configuration.

It was raised in GitHub issue #1233.
It could be backported to all stable branches.

(cherry picked from commit 18a82ba690a6ff4adbf9702cefa6dc89eb36372d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 4d8e20f83cc07c08c1d562e6afc5b4d3a54f4058)
[Cf: Patch applied on cfgparse-listen.c]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>