BUG/MINOR: hlua_fcn: ensure Patref:add_bulk() is given a table object before using it

BUG/MINOR: hlua_fcn: ensure Patref:add_bulk() is given a table object before using it

As reported by GH user @kanashimia in GH #3241, providing anything else
than a table to Patref:add_bulk() method could cause a segfault because
we were calling lua_next() with the lua object without ensuring it
actually is a table.

Let's add the missing lua_istable() check on the stack object before
calling lua_next() function on it.

It should be backported up to 3.2 with 884dc62 ("MINOR: hlua_fcn:
add Patref:add_bulk()")