Fix a rare case of incorrect UTF8 validation

fix #1245, fix #1249

The value used to fill the incomplete code point buffer is
changed to a character which no longer causes
utf8_checker::valid() to incorrectly return false.

doc/qbk/09_releases.qbk

@@ -9,6 +9,14 @@
 
 [section Release Notes]
 
+[heading Boost 1.69]
+
+[*Fixes]
+
+* ([issue 1245]) Fix a rare case of incorrect UTF8 validation
+
+[heading Boost 1.68]
+
 This version fixes a missing executor work guard in all composed operations
 used in the implementation. Users who are experiencing crashes related to
 asynchronous completion handlers are encouraged to upgrade. Also included
@@ -16,8 +24,6 @@ is an improved mechanism for generating random numbers used to mask outgoing
 websocket frames when operating in the client mode. This resolves a
 vulnerability described in the Beast Hybrid Assessment Report from Bishop Fox.
 
-[heading Boost 1.68]
-
 [*New Features]
 
 The include directory `<beast/experimental>` contains features which are not