libio: null terminate the buffer upon initial allocation in getdelim

libio: null terminate the buffer upon initial allocation in getdelim

Commit 33eff78c8b28adc4963987880e10d96761f2a167 caused issues in nbdkit
which had code similar to this to get the last line of the file:

    while (getline (&line, &len, fp) != -1)
      ;
    /* Process LINE.  */

After that commit, line[0] would be equal to '\0' instead of containing
the last line of the file like before that commit. A recent POSIX issue
clarified that the behavior before and after that commit are allowed,
since the contents of LINE are unspecified after -1 is returned
[1]. However, some programs rely on the previous behavior.

This patch null terminates the buffer upon getdelim/getline's initial
allocation. This is compatible with previous glibc versions, while also
protecting the caller from reading uninitialized memory if the file is
empty, as long as getline/getdelim does the initial allocation.

[1] https://www.austingroupbugs.net/bug_view_page.php?bug_id=1953

Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>