We need to initialize im_node to NULL, otherwise in case of error path
it gets passed to kfree() as uninitialized pointer.

Fixes: b95a5c4d ("bpf: add a longest prefix match trie map implementation")
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>