bpftool: Fix print error when show bpf map

bpftool: Fix print error when show bpf map

If there is no btf_id or frozen, it will not show the pids, but the pids don't
depend on any one of them.

Below is the result after this change:

  $ ./bpftool map show
  2: lpm_trie  flags 0x1
	key 8B  value 8B  max_entries 1  memlock 4096B
	pids systemd(1)
  3: lpm_trie  flags 0x1
	key 20B  value 8B  max_entries 1  memlock 4096B
	pids systemd(1)

While before this change, the 'pids systemd(1)' can't be displayed.

Fixes: 9330986c0300 ("bpf: Add bloom filter map implementation")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220320060815.7716-1-laoar.shao@gmail.com