BUG/MINOR: http: Missing calloc return value check while parsing tcp-request rule

BUG/MINOR: http: Missing calloc return value check while parsing tcp-request rule

A memory allocation failure happening in tcp_parse_request_rule while
processing the "capture" keyword and trying to allocate 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 8cb033643ff3235ac0d3887167ce06fefeaf850b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 31d170c581e8e4f726d7a54288b969cd38dde4b5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>