Check that the pointer to the string is not NULL for svn_utf__*_is_valid().

* subversion/libsvn_subr/utf_validate.c
  (svn_utf__cstring_is_valid, svn_utf__is_valid): Check data input before
    using it.  If it's NULL then return FALSE because it's not a valid
    string.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1455375 13f79535-47bb-0310-9956-ffa450edef68