bpftool: Fix pretty print dump for maps without BTF loaded

bpftool: Fix pretty print dump for maps without BTF loaded

The commit 15b0c9286085 ("bpftool: Use libbpf_get_error() to check error")
fails to dump map without BTF loaded in pretty mode (-p option).

Fixing this by making sure get_map_kv_btf won't fail in case there's
no BTF available for the map.

Fixes: 15b0c9286085 ("bpftool: Use libbpf_get_error() to check error")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220216092102.125448-1-jolsa@kernel.org