Fix issue 4339, diff suffix scanning invalid read at last chunk boundary.

* subversion/libsvn_diff/diff_file.c
  (find_identical_suffix): Handle last chunk.

* subversion/tests/libsvn_diff/diff-diff3-test.c
  (test_token_compare): Add comments, tweak allocation to match use (which
   doesn't change the underlying allocation due to alignment rounding).


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461590 13f79535-47bb-0310-9956-ffa450edef68