eliminate GCC warning for unchecked result of fread (#6167)

## Problem


GCCproduce warning that bread result is not checked. It doesn't affect
program logic, but better live without warnings.

## Summary of changes

Check read result.

## 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

pgxn/neon/walproposer_pg.c

@@ -1712,9 +1712,9 @@ walprop_pg_after_election(WalProposer *wp)
 	f = fopen("restart.lsn", "rb");
 	if (f != NULL && !wp->config->syncSafekeepers)
 	{
-		fread(&lrRestartLsn, sizeof(lrRestartLsn), 1, f);
+		size_t rc = fread(&lrRestartLsn, sizeof(lrRestartLsn), 1, f);
 		fclose(f);
-		if (lrRestartLsn != InvalidXLogRecPtr)
+		if (rc == 1 && lrRestartLsn != InvalidXLogRecPtr)
 		{
 			elog(LOG, "Logical replication restart LSN %X/%X", LSN_FORMAT_ARGS(lrRestartLsn));