selftests/bpf: Fix build error due to certain uninitialized variables

With the latest llvm21 compiler, I hit several errors when building bpf
selftests. Some of errors look like below:

  test_maps.c:565:40: error: variable 'val' is uninitialized when passed as a
      const pointer argument here [-Werror,-Wuninitialized-const-pointer]
    565 |         assert(bpf_map_update_elem(fd, NULL, &val, 0) < 0 &&
        |                                               ^~~

  prog_tests/bpf_iter.c:400:25: error: variable 'c' is uninitialized when passed
      as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
  400 |         write(finish_pipe[1], &c, 1);
      |                                ^

Some other errors have similar the pattern as the above.

These errors are fixed by initializing those variables properly.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250715185910.3659447-1-yonghong.song@linux.dev