Set nss[].lprefix to 0 when prefix is NULL in nad_(add/append)_namepsace

Code in nad.c, specifically nad_insert_nad(), checks lprefix to test if
an element needs to be prefixed. nad_append_namespace() called from
nad_parse() doesn't explicitly set lprefix to zero when prefix is NULL.
Since memory in nad.c is allocated via realloc(), newly allocated memory
is un-initialized and depending on your luck can be either 0 or non-zero
which can lead to phantom namespace prefixes being added.