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

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

A memory allocation failure happening in chash_init_server_tree while
trying to allocate a server's lb_nodes item used in consistent hashing
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 476462010ecc2329e9ecba5afd20f84d9605d809)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit c5c5bc4e36ce4a6f3bc113c8e16824fdb276c220)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>