Fix NPE when validating parent directory of a file

src/main/java/de/vanitasvitae/imi/codes/input/InputValidator.java

@@ -72,7 +72,7 @@ public class InputValidator {
      */
     public static File validateOutputPath(String path) throws InvalidOptionException {
         File file = new File(path);
-        File parent = file.getParentFile();
+        File parent = file.getAbsoluteFile().getParentFile();
         if (!parent.exists() || parent.isFile()) {
             throw new InvalidOptionException("Invalid directory " + parent.getAbsolutePath());
         }