Adding a stable MissingNo, and fixing a few minor things

engine/events/give_pokemon.asm

@@ -57,6 +57,8 @@ SetPokedexOwnedFlag:
 	ld [wd11e], a
 	predef IndexToPokedex
 	ld a, [wd11e]
+	and a
+	ret z ; do nothing for missingno
 	dec a
 	ld c, a
 	ld hl, wPokedexOwned

engine/gfx/mon_icons.asm

@@ -262,7 +262,6 @@ GetPartyMonSpriteID:
 	predef IndexToPokedex
 	ld a, [wd11e]
 	ld c, a
-	dec a
 	srl a
 	ld hl, MonPartyData
 	ld e, a

engine/items/item_effects.asm

@@ -540,6 +540,8 @@ ItemUseBall:
 ; Add the caught Pokémon to the Pokédex.
 	predef IndexToPokedex
 	ld a, [wd11e]
+	and a ; is it missingno?
+	jr z, .skipShowingPokedexData ; don't mark in pokedex if so
 	dec a
 	ld c, a
 	ld b, FLAG_TEST

engine/pokemon/add_mon.asm

@@ -86,6 +86,8 @@ _AddPartyMon::
 	predef IndexToPokedex
 	pop de
 	ld a, [wd11e]
+	and a
+	jr z, .noMarkSeen ; if it's missingno don't do any pokedex actions
 	dec a
 	ld c, a
 	ld b, FLAG_TEST
@@ -102,7 +104,7 @@ _AddPartyMon::
 	pop bc
 	ld hl, wPokedexSeen
 	call FlagAction
-
+.noMarkSeen
 	pop hl
 	push hl