Fix an instance where we call memcpy of 0 bytes with a NULL target.

* subversion/include/svn_delta.h
  (svn_txdelta_apply_instructions): Update docstring.

* subversion/libsvn_delta/text_delta.c
  (svn_txdelta_apply_instructions): Don't touch any of the buffers
                                    if the target has zero size.

Found by: philipm

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632635 13f79535-47bb-0310-9956-ffa450edef68