Do not panic when StdIn is closed

When the shell executing yq has no open stdin, os.Stdin.Stat() return nil and yq fails to continue. This commit fixes a missing verification on the result of os.Stdin.Stat() in the utils.processStdInArgs function and adds an acceptance test to cover this scenario in the future. This bug affects yq since version 4.26.1.

acceptance_tests/basic.sh

@@ -362,6 +362,12 @@ EOM
   assertEquals "$expected" "$X"
 }
 
-
+testBasicClosedStdIn() {
+  cat >test.yml <<EOL
+a: 1
+EOL
+  X=$(./yq e '.a' test.yml <&-)
+  assertEquals "1" "$X"
+}
 
 source ./scripts/shunit2

cmd/utils.go

@@ -227,8 +227,11 @@ func maybeFile(str string) bool {
 }
 
 func processStdInArgs(args []string) []string {
-	stat, _ := os.Stdin.Stat()
-	pipingStdin := (stat.Mode() & os.ModeCharDevice) == 0
+	stat, err := os.Stdin.Stat()
+	if err != nil {
+		yqlib.GetLogger().Debugf("error getting stdin: %v", err)
+	}
+	pipingStdin := err == nil && (stat.Mode()&os.ModeCharDevice) == 0
 
 	// if we've been given a file, don't automatically
 	// read from stdin.