Always handle POLLHUP in walredo error poll loop (#5716)

## Problem

test_stderr hangs on MacOS.

See https://neondb.slack.com/archives/C036U0GRMRB/p1698438997903919

## Summary of changes

Always handle POLLHUP to prevent infinite loop.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pageserver/src/walredo.rs

@@ -857,7 +857,8 @@ impl WalRedoProcess {
             let in_revents = stdin_pollfds[0].revents().unwrap();
             if in_revents & (PollFlags::POLLERR | PollFlags::POLLOUT) != PollFlags::empty() {
                 nwrite += proc.stdin.write(&writebuf[nwrite..])?;
-            } else if in_revents.contains(PollFlags::POLLHUP) {
+            }
+            if in_revents.contains(PollFlags::POLLHUP) {
                 // We still have more data to write, but the process closed the pipe.
                 anyhow::bail!("WAL redo process closed its stdin unexpectedly");
             }
@@ -907,7 +908,8 @@ impl WalRedoProcess {
                 let out_revents = stdout_pollfds[0].revents().unwrap();
                 if out_revents & (PollFlags::POLLERR | PollFlags::POLLIN) != PollFlags::empty() {
                     nresult += output.stdout.read(&mut resultbuf[nresult..])?;
-                } else if out_revents.contains(PollFlags::POLLHUP) {
+                }
+                if out_revents.contains(PollFlags::POLLHUP) {
                     anyhow::bail!("WAL redo process closed its stdout unexpectedly");
                 }
             }