bpftool: Fix memory leak in do_build_table_cb

strdup() allocates memory for path. We need to release the memory in the
following error path. Add free() to avoid memory leak.

Fixes: 8f184732

 ("bpftool: Switch to libbpf's hashmap for pinned paths of BPF objects")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221206071906.806384-1-linmq006@gmail.com