I just made the dist-upgrade to Debian 9.0, which included an update of package nano to version 2.7.4-1 and the problem vanished, the bug is solved in 2.7.4-1.
Yes, the bug is absent since version 2.3.2. It was fixed on 2013-01-02 by David Benjamin (git commit a016f00f). Later, the bug was avoided entirely by putting the linked list of syntaxes upside-down and not bothering to free a redefined syntax at all. Benno
