gdb/corelow.c: fix use-after-free in build_file_mappings

In core_target::build_file_mappings, GDB tries to open files referenced
in the core dump.

The process goes like this:

    struct bfd *bfd = bfd_map[filename];
    if (bfd == nullptr)
      {
        bfd = bfd_map[filename]
          = bfd_openr (expanded_fname.get (), "binary");
        if (bfd == nullptr || !bfd_check_format (bfd, bfd_object))
          {
            if (bfd != nullptr)
              bfd_close (bfd);
            return;
          }
      }
    asection *sec = bfd_make_section_anyway (bfd, "load");
    ...

The problem is that if bfd_check_format fails, we close the bfd but keep
a reference to it in the bfd_map.

If the same filename appears another time in the NT_FILE note, we enter
this code again.  The second time, bfd_map[filename] is not nullptr and
we try to call bfd_make_section_anyway on an already closed BFD, which
is a use-after-free error.

This patch makes sure that the bfd is only saved in the bfd_map if it
got opened successfully.

This error got exposed by a recent change in BFD (014a602b "Don't
optimise bfd_seek to same position").  Since this change, opening a
coredump which contains mapping to some special files such as a DRI
render node (/dev/dri/renderD$NUM) exposes the issue.  This happens for
example for processes using AMDGPU devices to offload compute tasks.

Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>