tools/bpf_jit_disasm: Fix potential negative tpath index in get_exec_path()

If readlink() fails, len will be -1, which can cause negative indexing
and undefined behavior. This patch ensures that len is set to 0 on
readlink failure, preventing such issues.

Signed-off-by: Ruslan Semchenko <uncleruc2075@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250612131816.1870-1-uncleruc2075@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>