BUG/MINOR: debug: make ha_task_dump() always check the task before dumping it

For now it cannot happen since we're calling it from a task but it will
break with signals. No backport is needed.
diff --git a/src/debug.c b/src/debug.c
index 54f91d6..47bf6e5 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -75,6 +75,11 @@
  */
 void ha_task_dump(const struct task *task, const char *pfx)
 {
+	if (!curr_task) {
+		chunk_appendf(&trash, "0\n");
+		return;
+	}
+
 	chunk_appendf(&trash,
 	              "%p (%s) calls=%u last=%llu%s\n",
 	              curr_task, TASK_IS_TASKLET(curr_task) ? "tasklet" : "task",