fix bug #22251 (map labels not appearing in mp)

This fixes a bug introduced in this commit:
90501cf

If the variable "res" is redeclared inside the else clause, the
desired return value goes out of scope before the function ends,
and we always return NULL instead.

This commit makes the line assign the variable without redeclaring
it.

src/map_label.cpp

@@ -183,7 +183,7 @@ const terrain_label* map_labels::set_label(const map_location& loc,
 		terrain_label* global_label = get_label_private(loc, "");
 
 		// Add the new label.
-		terrain_label* res = new terrain_label(text,
+		res = new terrain_label(text,
 				team_name,
 				loc,
 				*this,