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()")

src/hlua_fcn.c

@@ -2796,6 +2796,11 @@ static int _hlua_patref_add_bulk(lua_State *L, int status, lua_KContext ctx)
 	int count = 0;
 	int ret;
 
+	if (!lua_istable(L, 2)) {
+		luaL_argerror(L, 2, "argument is expected to be a table");
+		return 0; // not reached
+	}
+
 	if ((ref->flags & HLUA_PATREF_FL_GEN) &&
 	    pat_ref_may_commit(ref->ptr, ref->curr_gen))
 		curr_gen = ref->curr_gen;