stdin multiple file fixes

[yoniko]

Fixes for #3206 - bugs when handling stdin as part of multiple files.

[yoniko]

I haven't added tests for the cases where we fail on stdin/stdout being a console as our test suite actually doesn't use the console.
Here are the outputs of the manual checks:

❯ ./zstd ./file1 - -c
stdin is a console, aborting
❯ ./zstd - -c
stdin is a console, aborting
❯ zstd - -c
stdin is a console, aborting
❯ ./zstd ./file1 -
stdin is a console, aborting
❯ ./zstd ./file1 - -c
stdin is a console, aborting
❯ ./zstd - -c
stdin is a console, aborting
❯ ./zstd -
stdin is a console, aborting
❯ ./zstd - -f
aaaaa
(�/�X1aaaaa
B�%
❯ ./zstd - ./file1 -f
dwadawd
Compress: 1/2 files. Current: /*stdin*\ Read:     0   B  ==>  0%(�/�XAdwadawd
  2 files compressed :285.71%   (    14   B =>     40   B)
❯ echo "abc" | ./zstd -
stdout is a console, aborting
❯ echo "abc" | ./zstd - -c
(�/�X!abc
-nL�%
❯ echo "abc" | ./zstd ./file1 -
stdout is a console, aborting
❯ echo "abc" | ./zstd ./file1 - -c
(�/�$1file1
C0W(�/�X!abc
-nL�%

[terrelln]

LGTM

[terrelln]

nit: newline at end of file

[yoniko]

Fixed