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

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

A memory allocation failure happening during proxy_parse_declare while
processing the "capture" keyword and allocating a cap_hdr structure
would have resulted in a crash. This function is only called during
configuration parsing.

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

(cherry picked from commit 55ba0d6865662036d1d137dc4aac3841d29ad6d3)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 581d892c823ec18ff7066af696c2017c664959da)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>