tools/bpf_jit_disasm: silence a static checker warning

There is a static checker warning that "proglen" has an upper bound but
no lower bound.  The allocation will just fail harmlessly so it's not a
big deal.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>