honour length of packed keyword table

c64/basic/packed_messages.s

@@ -120,3 +120,4 @@ packed_keywords:
 	.byte $f6,$2f,$24,$00,$f2,$5b,$f9,$00
 	.byte $f1
 	.byte $30
+	.alias packed_keyword_table_len $DA

c64/basic/tokenise_line.s

@@ -137,7 +137,7 @@ next_kw_offset:
 	;; Advance offset in compressed token list, and stop at the end
 	inc tokenise_work3
 	lda tokenise_work3
-	cmp #$ff
+	cmp #packed_keyword_table_len
 	bne next_in_match
 	jmp done_searching_for_token
 next_in_match: