src: fix crash when inputting an incomplete set add command

src: fix crash when inputting an incomplete set add command

After inputting the following nft command, set->keytype is not initialized
but we try to destroy it, so NULL pointer dereference will happen:
  # nft add set t s
  Segmentation fault (core dumped)
  #0  dtype_free (dtype=0x0) at datatype.c:1049
  #1  set_datatype_destroy (dtype=0x0) at datatype.c:1051
  #2  0x0000000000407f1a in set_free (set=0x838790) at rule.c:213
  #3  0x000000000042ff70 in nft_parse (scanner=scanner@entry=0x8386a0,
    state=state@entry=0x7ffc313ea670) at parser_bison.c:9355
  #4  0x000000000040727d in nft_run (scanner=scanner@entry=0x8386a0,
    state=state@entry=0x7ffc313ea670, msgs=msgs@entry=0x7ffc313ea660)
    at main.c:237
  #5  0x0000000000406e4a in main (argc=<optimized out>, argv=<optimized
    out>) at main.c:376

Fixes: b9b6092304ae ("evaluate: store byteorder for set keys")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>