BUG/MINOR: worker: Missing calloc return value check in mworker_env_to_proc_list

BUG/MINOR: worker: Missing calloc return value check in mworker_env_to_proc_list

A memory allocation failure happening in mworker_env_to_proc_list when
trying to allocate a mworker_proc would have resulted in a crash. This
function is only called during init.

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

(cherry picked from commit 1f4fa906c73e61dba74f9e8b762da12df3052f57)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit d90aa347d2a3a81bae8461c009c9b4d69eb069a5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>