BUG/MINOR: mworker: don't use unitialized mworker_proc struct

If the reload fail after the parsing of the configuration, the
mworker_proc structures are created for the processes it tried to
create.

The mworker_proc_list_to_env() function was exporting these unitialized
structures in the "HAPROXY_PROCESSES" environment variable which was
leading to this kind of output in "show proc":

4294967295      worker          [was: 1]        1               17879d 16h26m28s
1 file changed