selftests/bpf: Fix a compilation warning

The following warning is triggered when I used clang compiler
to build the selftest.

  /.../prog_tests/btf_dedup_split.c:368:6: warning: variable 'btf2' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (!ASSERT_OK(err, "btf_dedup"))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /.../prog_tests/btf_dedup_split.c:424:12: note: uninitialized use occurs here
        btf__free(btf2);
                  ^~~~
  /.../prog_tests/btf_dedup_split.c:368:2: note: remove the 'if' if its condition is always false
        if (!ASSERT_OK(err, "btf_dedup"))
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /.../prog_tests/btf_dedup_split.c:343:25: note: initialize the variable 'btf2' to silence this warning
        struct btf *btf1, *btf2;
                               ^
                                = NULL

Initialize local variable btf2 = NULL and the warning is gone.

Fixes: 9a49afe6

 ("selftests/bpf: Add btf_dedup case with duplicated structs within CU")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211209050403.1770836-1-yhs@fb.com