When the stdout stream gets broken, there is no point in reading

When the stdout stream gets broken, there is no point in reading
any more input files, and it would be misleading to start a parser,
because that would show randomly truncated text.
Instead, print an error message and exit the program.

Issue found by Leah Neukirchen <leah at vuxu dot org>, who was
surprised to see half a manpage when her /tmp/ overflew.