iso9660: Fail when expected Rockridge extensions is missing

A corrupted or malicious ISO9660 image can cause read_CE() to loop
forever.

read_CE() calls parse_rockridge(), expecting a Rockridge extension
to be read. However, parse_rockridge() is structured as a while
loop starting with a sanity check, and if the sanity check fails
before the loop has run, the function returns ARCHIVE_OK without
advancing the position in the file. This causes read_CE() to retry
indefinitely.

Make parse_rockridge() return ARCHIVE_WARN if it didn't read an
extension. As someone with no real knowledge of the format, this
seems more apt than ARCHIVE_FATAL, but both the call-sites escalate
it to a fatal error immediately anyway.

Found with a combination of AFL, afl-rb (FairFuzz) and qsym.