BUG/MEDIUM: lua: handle stick table implicit arguments right.

BUG/MEDIUM: lua: handle stick table implicit arguments right.

In hlua_lua2arg_check(), we allow for the first argument to not be
provided, if it has a type we know, this is true for frontend, backend,
and stick table. However, the stick table code was changed. It used
to be deduced from the proxy, but it is now directly provided in struct
args. So setting the proxy there no longer work, and we have to
explicitely set the stick table.
Not doing so will lead the code do use the proxy pointer as a stick
table pointer, which will likely cause crashes.

This should be backported up to 2.0.