preparation towardfs string concatenation

src/basic/engine/varstr_garbage_collect.s

@@ -289,6 +289,22 @@ varstr_garbage_collect_check_bptr:
 	lda __FAC1+2
 	adc INDEX+1
 	sta __FAC1+2
+!:
+	// Same for __FAC2
+
+	lda __FAC2+2
+	cmp TXTPTR+1
+	bne !+
+	lda __FAC2+1
+	cmp TXTPTR+0
+	bne !+
+
+	clc
+	adc INDEX+0
+	sta __FAC2+1
+	lda __FAC2+2
+	adc INDEX+1
+	sta __FAC2+2
 !:
 	// Next iteration