Correct wTileMapBackup declared space

wTileMapBackup isnt just used to backup the tilemap but also as a buffer for a screen view out of blocks during the drawing of the tilemap

ram/wram.asm

@@ -153,13 +153,14 @@ wShadowOAMEnd::
 
 SECTION "Tilemap", WRAM0
 
+; buffer to load the screen view from blocks during LoadCurrentMapView (6*5 blocks worth of tiles)
 ; buffer for tiles that are visible on screen (20 columns by 18 rows)
 wTileMap:: ds SCREEN_WIDTH * SCREEN_HEIGHT
 
 UNION
 ; buffer for temporarily saving and restoring current screen's tiles
 ; (e.g. if menus are drawn on top)
-wTileMapBackup:: ds SCREEN_WIDTH * SCREEN_HEIGHT
+wTileMapBackup:: ds 24 * 20
 
 NEXTU
 ; list of indexes to patch with SERIAL_NO_DATA_BYTE after transfer
@@ -169,8 +170,6 @@ wSerialPartyMonsPatchList:: ds 200
 wSerialEnemyMonsPatchList:: ds 200
 ENDU
 
-	ds 80
-
 
 SECTION "Overworld Map", WRAM0