Fix a case of reading beyond allocated memory with SVN0, i.e. pre-1.4,
FSFS repositories.  This was identified by running svnadmin_tests.py 14
over ra_svn with svnserve running under valgrind.

* subversion/libsvn_delta/svndiff.c
  (decode_window): Allocate, copy and add terminating null for SVN0, remove
   incorrect comment for SVN1.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1587968 13f79535-47bb-0310-9956-ffa450edef68