Fix a SEGV triggered by commiting a change to a repository file
without a checksum (one created by svn_fs_make_file without a
subseqent svn_fs_apply_textdelta).

* subversion/libsvn_fs_base/tree.c
  (txn_body_apply_textdelta): Allow retreived checksum to be NULL.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464413 13f79535-47bb-0310-9956-ffa450edef68