TSG S4: improve magican's movement behaviour

- the movement restriction won't be applied if the magican is not
  in the caslte (due to the one exception).
- As for the exception, it's not enough if the magican has only one
  unit left, but this unit has to be on the caslte.

To sum up, the magican never leaves the castle, unless he has
(before recruiting) one last unit which is on the castle. (Maybe
from last turn's recruiting).
In case he left the castle, he goes back next to the keep next turn,
as leaders usually do.

[ci skip]

(cherry-picked from commit d4a6df0)

data/campaigns/The_South_Guard/scenarios/04_Vale_of_Tears.cfg

@@ -160,7 +160,7 @@
     [event]
         name=side 2 turn refresh
         first_time_only=no
-        # Exception to avoid the leader being passive even after he is basically defeated.
+        # Exception to avoid the leader being passive even after his army is defeated.
         # If the leader recruited one unit last turn, they can leave the castle together.
         [filter_condition]
             [not]
@@ -169,6 +169,14 @@
                     canrecruit=no
                     count=1
                 [/have_unit]
+                [have_unit]
+                    side=2
+                    canrecruit=no
+                    [filter_location]
+                        terrain=C*^*
+                    [/filter_location]
+                    count=1
+                [/have_unit]
             [/not]
         [/filter_condition]
 
@@ -178,6 +186,11 @@
             take_only_once=no
             [filter]
                 id="Mal A'kai"
+                # Apply the movement restriction only if he is in the keep or castle,
+                # so he can still go back if he left due to the exception above.
+                [filter_location]
+                    terrain=K*^*,C*^*
+                [/filter_location]
             [/filter]
             [effect]
                 apply_to=movement_costs