fix bug when expanding last line on screen

c64/kernal/chrout.s

@@ -226,6 +226,10 @@ count_rows_loop:
 	cpy current_screen_y
 	bne count_rows_loop
 
+	stx $0420
+	cpx #0
+	beq no_copy_down
+	
 	;; Set pointers to end of screen line, and one line
 	;; above.  (It is always one physical line, because
 	;; this can only happen when expanding a line from 40 to
@@ -293,6 +297,7 @@ cl_inner:
 	dex
 	bne copy_line_down_loop
 
+no_copy_down:	
 	jsr calculate_screen_line_pointer
 
 	;; Erase newly inserted line