[Lua] Fix some accidental global warnings

data/ai/lua/battle_calcs.lua

@@ -878,7 +878,7 @@ function battle_calcs.attack_rating(attacker, defender, dst, cfg, cache)
     -- If defender is on a healing location, count that as slightly more than the healing amount
     def_damage = def_damage - 1.25 * wesnoth.terrain_types[map[defender]].healing
 
-    if (def_damage < 0) then damage = 0. end
+    if (def_damage < 0) then def_damage = 0. end
 
     -- Fraction damage (= fractional value of the unit)
     local def_value_fraction = def_damage / defender.max_hitpoints

data/ai/micro_ais/micro_ai_helper.lua

@@ -21,7 +21,7 @@ local function is_number(value, check_int, ai_type, name)
         if (number ~= math.floor(number)) then
             return false
         elseif (math.type(number) ~= 'integer') then
-            str = "[micro_ai] tag (" .. ai_type .. ") parameter '" .. name .. "' must be an integer. It has an integer representation, but is provided in floating-point format."
+            local str = "[micro_ai] tag (" .. ai_type .. ") parameter '" .. name .. "' must be an integer. It has an integer representation, but is provided in floating-point format."
             warn(str)
             std_print(str .. ' (see Lua console for stack trace)')
         end
@@ -365,7 +365,7 @@ function micro_ai_helper.micro_ai_setup(cfg, CA_parms, required_keys, optional_k
                 end
             end
             if is_invalid then
-                str = "[micro_ai] tag (" .. cfg.ai_type .. ") contains invalid parameter: [" .. t[1] .. "]"
+                local str = "[micro_ai] tag (" .. cfg.ai_type .. ") contains invalid parameter: [" .. t.tag .. "]"
                 warn(str)
                 std_print(str .. ' (see Lua console for stack trace)')
             end

data/lua/on_event.lua

@@ -31,7 +31,7 @@ return function(eventname, priority, fcn)
 		priority = priority,
 		first_time_only = false,
 		action = function()
-			context = wesnoth.current.event_context
+			local context = wesnoth.current.event_context
 			wesnoth.experimental.game_events.set_undoable(true)
 			fcn(context)
 		end