tools/bpftool: Fix segfault case regarding 'pin' arguments

Arguments of 'pin' subcommand should be checked
at the very beginning of do_pin_any().
Otherwise segfault errors can occur when using
'map pin' or 'prog pin' commands, so fix it.

  # bpftool prog pin id
  Segmentation fault

Fixes: 71bb428f

 ("tools: bpf: add bpftool")
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>