figments: ringbuf: prevent infinite loop if ringbuf is empty

lib/Figments/Ringbuf.h

@@ -86,6 +86,8 @@ struct Ringbuf {
     size_t size() {
       if (m_tail > m_head) {
         return m_tail - m_head;
+      } else if (m_tail == m_head) {
+        return 0;
       }
       return m_tail + (Size - m_head);
     }