BUG/MEDIUM: stats: allocate the stats frontend also on "stats bind-process"

Bryan Talbot reported that a config with only "stats bind-process 1" in
the global section would crash during parsing. This bug was introduced
with this new statement in 1.5-dev13. The stats frontend must be allocated
if it was not yet.

No backport is needed. The workaround consists in having previously declared
any other stats keyword (generally stats socket).