tools: bpf_jit_disasm: check for klogctl failure

klogctl can fail and return -ve len, so check for this and
return NULL to avoid passing a (size_t)-1 to malloc.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>