[BASIC] Prevent OLD command from hanging when run without a program i…

…n memory (X16Community#237)

Should also prevent a needless freeze when loading a non-BASIC program into low ram by mistake

* Initialize a couple of memory locations to zero to prevent OLD from hanging

* Remove changes as they do not work on reset

* Ensore OLD function exits if index rolls over.
This is to solve the issue where OLD would hang if there is no program
present in memory.
If index rollover is detected, 0's will be written to the two first
bytes of txttab ($0801) to indicate that there is no BASIC program
in memory.

* Removing commented abandoned approach

* Clarify comments, save a byte

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Co-authored-by: mooinglemur <[email protected]>

basic/code1.s

@@ -200,7 +200,13 @@ chead	ldy #1
 	beq lnkrts
 	ldy #4
 czloop	iny
-	lda (index),y
+	bne :+		; Continue as normal as long as Y has not rolled over to 0
+	tya		; If Y has rolled over to 0, we have encountered a line > 255 bytes.
+	sta (index),y	; store 0's in the next two bytes
+	iny		; to indicate the end of the BASIC program
+	sta (index),y
+	rts
+:	lda (index),y
 	bne czloop
 	iny
 	tya