On the authzperf branch: Follow-up to r1615410.

* subversion/libsvn_subr/string.c
  (svn_stringbuf_strip_whitespace): Fix off-by-one.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/authzperf@1615421 13f79535-47bb-0310-9956-ffa450edef68