Fix a newly introduced segfault in callback.c.

Colin Harrison points out that commit c31e3cd43 was less cautious than
it should have been: when delete_callbacks_for_context nulls out the
'next' pointer in the new tail element of the callbacks list, it
should only do so if there _is_ a new tail element. If the list has
just become empty, that won't work very well!

callback.c

@@ -65,6 +65,7 @@ void delete_callbacks_for_context(void *ctx)
 
     cbhead = newhead;
     cbtail = newtail;
+    if (newtail)
         newtail->next = NULL;
 }