On the fsfs-format7 branch: protect svn__base36toui64 against a potential
buffer overflow caused by overly long number representations.

Found by: cmpilato

* subversion/include/private/svn_string_private.h
  (svn__base36toui64): mention overflow behaviour in docstring

* subversion/libsvn_subr/string.c
  (svn__base36toui64): make sure we never overflow our internal buffer

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1454307 13f79535-47bb-0310-9956-ffa450edef68