Make svn_stringbuf_appendbytes(..., NULL, 0) be well defined by
avoiding passing NULL to memcpy. GCC's undefined behaviour sanitizer
detects that we do this during skel unparsing.

* subversion/include/svn_string.h
  (svn_stringbuf_appendbytes): Document behaviour.

* subversion/libsvn_subr/string.c
  (svn_stringbuf_appendbytes): Return early.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632530 13f79535-47bb-0310-9956-ffa450edef68