Checkmk Werk 12707: `md` check: crashes with IndexError (list index out of range) on invalid input

ID: 12707 Title: `md` check: crashes with IndexError (list index out of range) on invalid input Component: Checks & agents Level: 1 Class: Bug fix Version: 2.1.0i1 Fed with input different from what the `md` check is made for it tries to access non-existent input elements and crashes with unhandled IndexError exception. This change makes `md` check for the minimum number of expected elements before accessing them.