Check unit/second_unit existence in FORCE_CHANCE_TO_HIT

Fixes an issue where the auxiliary event handlers for 'attack end' in
this macro might break if the affected unit or units are removed from
the map by a pre-existing 'attack end' event handler.

Unfortunately this still results in log warnings about failing to
auto-store unit/second_unit, but it's better than breaking WML execution
entirely.

    20190313 23:36:48 error engine: failed to auto-store $second_unit at (9,40)

The alternative I contemplated was to force unit/second_unit to be valid
in the event handlers by using empty filters, but then that would result
in the event handler being triggered *later* by another valid unit. This
is especially problematic since the event handler handles special
knowledge about the unit's WML ([specials][original_chance_to_hit]).
That option would not produce any spurious error messages but would
definitely cause new problems, especially when interacting with other
FCTH substitutions in the same scenario.

Ultimately FCTH's design is a bit questionable and relies too much on
state that may be broken by an external agent intentionally or otherwise
and produce unusual results.

Closes #3982.

[ci skip]

changelog.md

@@ -52,6 +52,9 @@
    * Fix undoing a recall not un-drawing parts of the sprite that go beyond the unit's hex (issue #3325)
    * Fix crash when the recruit filter matched nothing. (PR #3969)
    * "Show Enemy Moves" now highlights enemy units that can reach the highlighted hex. (PR #3961)
+ ### Miscellaneous and bug fixes
+   * Fix auxiliary attack end event handlers defined by FORCE_CHANCE_TO_HIT breaking if one or more of the
+     affected units are removed by another attack end event handler (issue #3982).
 
 ## Version 1.14.6
  ### AI

data/core/macros/utils.cfg

@@ -578,6 +578,17 @@
             name=attack end
             delayed_variable_substitution=yes
 
+            [if]
+                [variable]
+                    name=unit.length
+                    numerical_equals=0
+                [/variable]
+                [then]
+                    # Unit vanished mid-fight
+                    [return][/return]
+                [/then]
+            [/if]
+
             [foreach]
                 array=unit.attack
                 [do]
@@ -665,6 +676,17 @@
             name=attack end
             delayed_variable_substitution=yes
 
+            [if]
+                [variable]
+                    name=second_unit.length
+                    numerical_equals=0
+                [/variable]
+                [then]
+                    # Unit vanished mid-fight
+                    [return][/return]
+                [/then]
+            [/if]
+
             [foreach]
                 array=second_unit.attack
                 [do]