Follow-up to r1436303, fix build problem.

* subversion/libsvn_client/patch.c
  (readline): The stringbuf_t length field is called 'len', not 'size'.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436309 13f79535-47bb-0310-9956-ffa450edef68