Fix: Check the stdin scanner for errors when reading the password

Reading from stdin with Scanner.Scan() can either fail because of a read
error, or return no bytes because the EOF was reached.

This adds support for checking these cases before actually reading the
password.

cmd/sojuctl/main.go

@@ -120,7 +120,12 @@ func readPassword() ([]byte, error) {
 	} else {
 		fmt.Fprintf(os.Stderr, "Warning: Reading password from stdin.\n")
 		scanner := bufio.NewScanner(os.Stdin)
-		scanner.Scan()
+		if !scanner.Scan() {
+			if err := scanner.Err(); err != nil {
+				log.Fatalf("failed to read password from stdin: %v", err)
+			}
+			log.Fatalf("failed to read password from stdin: stdin is empty")
+		}
 		password = scanner.Bytes()
 
 		if len(password) == 0 {