Fix "uninitialized memory access" warning in valgrind.

* subversion/libsvn_fs_x/string_table.c
  (create_table): Make sure that even the padding area gets set the a
                  defined state and fix the docstring mess.

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573350 13f79535-47bb-0310-9956-ffa450edef68