Fixed loading of savegames where some unit has ACTIVITY_IRIGATE or ACTIVITY_MINE
without target.

Reported by Sveinung Kvilhaugsvik <sveinung>

See bug #21857


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24737 a0f10bec-cc02-0410-94fc-a9cfff90b4cd