libbpf: Workaround -Wmaybe-uninitialized false positive

In `elf_close`, we get this with GCC 15 -O3 (at least):
```
In function ‘elf_close’,
    inlined from ‘elf_close’ at elf.c:53:6,
    inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:57:9: warning: ‘elf_fd.elf’ may be used uninitialized [-Wmaybe-uninitialized]
   57 |         elf_end(elf_fd->elf);
      |         ^~~~~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.elf’ was declared here
  377 |         struct elf_fd elf_fd;
      |                       ^~~~~~
In function ‘elf_close’,
    inlined from ‘elf_close’ at elf.c:53:6,
    inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:58:9: warning: ‘elf_fd.fd’ may be used uninitialized [-Wmaybe-uninitialized]
   58 |         close(elf_fd->fd);
      |         ^~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.fd’ was declared here
  377 |         struct elf_fd elf_fd;
      |                       ^~~~~~
```

In reality, our use is fine, it's just that GCC doesn't model errno
here (see linked GCC bug). Suppress -Wmaybe-uninitialized accordingly
by initializing elf_fd.fd to -1 and elf_fd.elf to NULL.

I've done this in two other functions as well given it could easily
occur there too (same access/use pattern).

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://gcc.gnu.org/PR114952
Link: https://lore.kernel.org/bpf/14ec488a1cac02794c2fa2b83ae0cef1bce2cb36.1723578546.git.sam@gentoo.org

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