bpftool: Fix uninit variable compilation warning

Newer GCC complains about capturing the address of unitialized variable.
While there is nothing wrong with the code (the variable is filled out
by the kernel), initialize the variable anyway to make compiler happy.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220202225916.3313522-4-andrii@kernel.org