Fix some frontier gfx, adjacent cleanup

src/battle_tower.c

@@ -3175,7 +3175,7 @@ void CalcApprenticeChecksum(struct Apprentice *apprentice)
     s32 i;
 
     apprentice->checksum = 0;
-    for (i = 0; i < (sizeof(struct Apprentice) - 4) / 4; i++)
+    for (i = 0; i < offsetof(struct Apprentice, checksum) / sizeof(u32); i++)
         apprentice->checksum += ((u32 *)apprentice)[i];
 }
 
@@ -3183,7 +3183,7 @@ static void ClearApprentice(struct Apprentice *apprentice)
 {
     s32 i;
 
-    for (i = 0; i < (sizeof(struct Apprentice)) / 4; i++)
+    for (i = 0; i < sizeof(struct Apprentice) / sizeof(u32); i++)
         ((u32 *)apprentice)[i] = 0;
     ResetApprenticeStruct(apprentice);
 }
@@ -3196,7 +3196,7 @@ static void ValidateApprenticesChecksums(void)
     {
         u32 *data = (u32 *) &gSaveBlock2Ptr->apprentices[i];
         u32 checksum = 0;
-        for (j = 0; j < (sizeof(struct Apprentice) - 4) / 4; j++)
+        for (j = 0; j < offsetof(struct Apprentice, checksum) / sizeof(u32); j++)
             checksum += data[j];
         if (gSaveBlock2Ptr->apprentices[i].checksum != checksum)
             ClearApprentice(&gSaveBlock2Ptr->apprentices[i]);
@@ -3506,7 +3506,7 @@ bool32 ValidateBattleTowerRecord(u8 recordId) // unused
     u32 *record = (u32 *)(&gSaveBlock2Ptr->frontier.towerRecords[recordId]);
     u32 checksum = 0;
     u32 hasData = 0;
-    for (i = 0; i < (sizeof(struct EmeraldBattleTowerRecord) - 4) / 4; i++) // - 4, because of the last fjeld bejng the checksum jtself.
+    for (i = 0; i < offsetof(struct EmeraldBattleTowerRecord, checksum) / sizeof(u32); i++)
     {
         checksum += record[i];
         hasData |= record[i];