Fixing prototype process crash.

A prototype stores linked application processes structures.  When an
application process terminates, it's removed from the list.  To avoid double
removal, the pointer to the next element should be set to NULL.

The issue was introduced in c8790d2a89bb.

src/nxt_application.c

@@ -1150,6 +1150,8 @@ nxt_proto_process_remove(nxt_task_t *task, nxt_pid_t pid)
         process = lhq.value;
 
         nxt_queue_remove(&process->link);
+        process->link.next = NULL;
+
         break;
 
     default: