[BUG] reading from FIFO broken

As of this commit:

> commit a065467315dc5f2f312f7e6f1d2e9d05a77b3cb7
> Author: Herbert Xu <[email protected]>
> Date:   Sun Jun 2 09:31:32 2024 +0800
> 
>     input: Move newline loop into preadbuffer
>     
>     As it stands preadfd tries to fetch a whole line when we're reading
>     one byte at a time.  However, this is wrong because how many bytes
>     we're trying to read has nothing to do with whether we get a whole
>     line or not.
>     
>     Move the loop into preadbuffer alongside the other history support
>     code.
>     
>     Signed-off-by: Herbert Xu <[email protected]>

...reading from a FIFO is broken, at least on macOS 12.7.5. It seems like the 
'read' command can no longer read anything from a FIFO.

Reproducer:

$ dash -c 'trap "rm fifo" 0; mkfifo fifo;
	echo OK >fifo & read r <fifo; echo "${#r}:$r"'
0:

Expected output:

2:OK

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