Fixed script command getunits (#4417)

* Fixes #4416.
* Resolves an issue with script command getunits returning failure on a valid map index.
Thanks to @javirst and @sader1992!

src/map/script.cpp

@@ -11288,7 +11288,7 @@ BUILDIN_FUNC(getunits)
 	int type = script_getnum(st, 2);
 	int size = 0;
 	int32 idx, id;
-	int16 m = 0, x0 = 0, y0 = 0, x1 = 0, y1 = 0;
+	int16 m = -1, x0 = 0, y0 = 0, x1 = 0, y1 = 0;
 	struct s_mapiterator *iter = mapit_alloc(MAPIT_NORMAL, bl_type(type));
 
 	if (!strcmp(command, "getmapunits"))
@@ -11347,7 +11347,7 @@ BUILDIN_FUNC(getunits)
 
 	for (bl = (struct block_list*)mapit_first(iter); mapit_exists(iter); bl = (struct block_list*)mapit_next(iter))
 	{
-		if (!m || (m == bl->m && !x0 && !y0 && !x1 && !y1) || (bl->m == m && (bl->x >= x0 && bl->y >= y0) && (bl->x <= x1 && bl->y <= y1)))
+		if (m == -1 || (m == bl->m && !x0 && !y0 && !x1 && !y1) || (bl->m == m && (bl->x >= x0 && bl->y >= y0) && (bl->x <= x1 && bl->y <= y1)))
 		{
 			if (data)
 				set_reg(st, sd, reference_uid(id, idx + size), name, (is_string_variable(name) ? (void*)status_get_name(bl) : (void*)__64BPRTSIZE(bl->id)), reference_getref(data));