Fix a bug in memset.

If the upper byte of the int argument was nonzero, it could write the wrong value (the OR of the upper and lower bytes). It should convert the value to unsigned char, i.e. just use the lower byte.

string.asm

@@ -482,21 +482,19 @@ rtl	equ	1	return address
 
 	ph4	p	save the pointer
 
+	short	M
 	lda	val	form a 2 byte value
-	xba
-	ora	val
-	sta	val
+	sta	val+1
 
 	lda	len	if there are an odd # of bytes then
 	lsr	A
 	bcc	lb1
-	short M	  set 1 byte now
-	lda	val
+	lda	val	  set 1 byte now
 	sta	[p]
 	long	M
 	dec	len
 	inc4	p
-lb1	anop		endif
+lb1	long	M	endif
 
 	lda	val	set len bytes
 	ldx	len+2	set full banks