common: Fix a potential cycle in the trace structure

It turns out that under some circumstances we end up clearing the
pointee of `current` but not the pointer. Thus when we select the next
slot we can end up reusing the same slot, making it its own parent.

We forcefull break these cycles by enforcing that `current` should
never be returned and be set as its own parent.

Changelog-None

common/trace.c

@@ -179,6 +179,13 @@ static struct span *trace_span_slot(void)
 	 * concurrent spans. */
 	assert(s);
 	assert(s->parent == NULL);
+
+	/* Be extra careful not to create cycles. If we return the
+	 * position that is pointed at by current then we can only
+	 * stub the trace by removing the parent link here. */
+	if (s == current)
+		current = NULL;
+
 	return s;
 }