Revision 1589570

Merge r1587968 from trunk:

 * r1587968
   Maintain data[len]=0 invariant for svn_string_t.
   Justification:
     Code may rely on null byte being present.
   Notes:
     Triggers a read beyond allocated memory on trunk but no known
     misbehaviour on 1.8.
   Votes:
     +1: philip, rhuijben, stefan2