bug#49925: cat -E interprets sentinel newline at the end of buffer as an actual newline after a \r

[Michael Debertol]

Hi,

after https://lists.gnu.org/archive/html/coreutils/2021-02/msg00003.html 
(unreleased), the behavior of cat -E was changed so that it prints "^M$" 
for "\r\n" line endings.

Whenever it sees a \r "cat -E" checks if the byte after is a \n, however 
that \n might be the sentinel value that is inserted at the end of a buffer.

This is a problem in two cases:

- When a \r is at the end of the input. `printf "\r" | cat -E` will 
print "^M", even though there is no "\n" after the "\r". FWIW, 
tests/misc/cat-E.sh expects a "^M" for a trailing "\r", but I think 
that's wrong.

- When the file is too big to fit into one buffer. If you try to "cat 
-E" a big file (mutliple megabytes) that consists of only "\r", cat will 
print a few "^M" whenever it hits the end of a buffer in the middle of 
the file and at the end.

Michael