Minor optimization in svn_membuf code.
Minor optimization in svn_membuf code.

* subversion/libsvn_subr/string.c
  (membuf_create): always allocate, even 0 bytes
  (svn_membuf__resize): new allocated membuf->data can never be NULL

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1532186 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion