diff: Report I/O errors in Stone algorithm

[des]

Summary

In the legacy Stone algorithm, we do a first pass over the files to
check if they're identical before we start diffing them. That code
would correctly set the exit status if an I/O error was encountered,
but would not emit an error message. Do so.

PR: 292198
Fixes: 55f160fb07ea ("diff: Print an error message on I/O error")
MFC after: 1 week
Sponsored by: Klara, Inc.

[olce]

usr.bin/diff/diffreg.c

506–509

Putting the assignments in the if makes things less readable, especially given that i and j are not used in the test nor if blocks.

You might want to rename i and j for better clarity.