MB-11280 Fix Upr flow control msg parsing

Make sure value is a null-terminated string before converting.

Change-Id: I6528b548fb837d1974e296fc5e7fb84993acca7d
Reviewed-on: http://review.couchbase.org/37795
Tested-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>