Fix ChanContext backtrack matching with GPOS
Reported on mailing list by Keith Stribley and Khaled Hosny.
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index f50d87b..04ae8c9 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -45,7 +45,7 @@
*
* As an optimization, both info and out_info may point to the
* same piece of memory, which is owned by info. This remains the
- * case as long as out_len doesn't exceed len at any time.
+ * case as long as out_len doesn't exceed i at any time.
* In that case, swap() is no-op and the glyph operations operate
* mostly in-place.
*
