kernel/pipe: squash compiler warning

The compiler complains that:

```
zephyr/kernel/include/kernel_internal.h:121:29:
error: 'reader' may be used uninitialized [-Werror=maybe-uninitialized]
  121 |         thread->swap_retval = value;
      |         ~~~~~~~~~~~~~~~~~~~~^~~~~~~
zephyr/kernel/pipe.c: In function 'copy_to_pending_readers':
zephyr/kernel/pipe.c:92:26: note: 'reader' was declared here
   92 |         struct k_thread *reader;
      |                          ^~~~~~
```

The static analyzer fails to see through the `LOCK_SCHED_SPINLOCK`
construct that the `reader` pointer is always initialized.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>