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

This fixes a bug introduced in this commit:
90501cf87254d3d87f876beb504f8734535d1dea

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.
update changelog
Merge branch '1.12' of https://github.com/wesnoth/wesnoth into 1.12

Conflicts:
	changelog