Initialize DamageContext struct with zero values

Also forgot to initialize the struct in `Cmd_damagecalc`

src/battle_script_commands.c

@@ -1806,7 +1806,7 @@ static void Cmd_damagecalc(void)
 
     u32 moveTarget = GetBattlerMoveTargetType(gBattlerAttacker, gCurrentMove);
 
-    struct DamageContext ctx;
+    struct DamageContext ctx = {0};
     ctx.battlerAtk = gBattlerAttacker;
     ctx.move = gCurrentMove;
     ctx.moveType = GetBattleMoveType(gCurrentMove);