tail: fix uninitialized memory read when failing to read file

tail: fix uninitialized memory read when failing to read file

Reproduced under UBSAN with `tail -f <&-` giving:
  tail.c:2220:18: runtime error: load of value 190,
  which is not a valid value for type ‘_Bool'

* src/tail.c (tail_file): Ensure f->ignore is initialized
in all cases where we can't tail the specified file.
* tests/tail-2/follow-stdin.sh: Add a test case which
checks stderr has no UBSAN warnings.
Fixes http://bugs.gnu.org/25041