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: https://svn.apache.org/repos/asf/subversion/trunk@1532186 13f79535-47bb-0310-9956-ffa450edef68