stdin multiple file fixes (#3222)

* Fixes for https://github.com/facebook/zstd/issues/3206 - bugs when handling stdin as part of multiple files.

* new line at end of multiple-files.sh

tests/cli-tests/compression/multiple-files.sh.stdout.exact

@@ -0,0 +1,12 @@
+Test zstd ./file1 - file2
+stdin
+file1
+file2
+Test zstd -d ./file1.zst - file2.zst
+stdin
+file1
+file2
+zstd -d ./file1.zst - file2.zst -c
+file1
+stdin
+file2