selftests/bpf: Fix two minor compilation warnings reported by GCC 4.9

GCC 4.9 seems to be more strict in some regards. Fix two minor issue it
reported.

Fixes: 1c1052e0 ("tools/testing/selftests/bpf: Add self-tests for new helper bpf_get_ns_current_pid_tgid.")
Fixes: 2d7824ff ("selftests: bpf: Add test for sk_assign")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200820061411.1755905-3-andriin@fb.com