Don't allow player movement off-screen

src/player.s

@@ -53,16 +53,27 @@ Player_Update::
     and BTN_RIGHT
     jr z, .left
 
+    ; check for right boundary
+    ld hl, _OAM + 1
+    ld a, [hl]
+    cp 160
+    jr nc, .done
+
     ld hl, _OAM + 1
     inc [hl]
 
 .left:
-    ; check for move left
+    ; check for left button
     ld a, b
     and BTN_LEFT
     jr z, .done
 
+    ; check for left boundary
     ld hl, _OAM + 1
+    ld a, [hl]
+    cp 9
+    jr c, .done
+
     dec [hl]
 
 .done: