Reapply 016919a53b

Author: James Crook <james.k.crook@gmail.com>
   Date:   Sun Jun 26 08:35:19 2016 +0100

       Bug1223: (correction).  Fix new potential crash in following pointer.

       With recent changes to the Nyquist code for freeing blocks, the pointer 'next' could be uninitialised when 'list' points to the zero chain.  So Audacity would follow a rogue pointer.  We'd get away with it if the uninitialised value happened to be zero.

lib-src/libnyquist/nyquist/nyqsrc/sound.c

@@ -545,6 +545,7 @@ void snd_list_unref(snd_list_type list)
             break; // the rest of the list is shared, nothing more to free
         }
 
+        next = NULL;
         // list nodes either point to a block of samples or this is the 
         // last list node (list->block == NULL) which points to a suspension
         // lists can also terminate at the zero_block, which is an infinite