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

@@ -0,0 +1,21 @@
+#!/bin/sh
+set -e
+
+# setup
+echo "file1" > file1
+echo "file2" > file2
+
+echo "Test zstd ./file1 - file2"
+rm -f ./file*.zst
+echo "stdin" | zstd ./file1 - ./file2 | zstd -d
+cat file1.zst | zstd -d
+cat file2.zst | zstd -d
+
+echo "Test zstd -d ./file1.zst - file2.zst"
+rm ./file1 ./file2
+echo "stdin" | zstd - | zstd -d ./file1.zst - file2.zst
+cat file1
+cat file2
+
+echo "zstd -d ./file1.zst - file2.zst -c"
+echo "stdin" | zstd | zstd -d ./file1.zst - file2.zst -c

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