primitives/ringbuf_queue: Fix bug in init.

Author: peterhinch

v3/primitives/ringbuf_queue.py

@@ -14,7 +14,7 @@
 class RingbufQueue:  # MicroPython optimised
     def __init__(self, buf):
         self._q = [0 for _ in range(buf)] if isinstance(buf, int) else buf
-        self._size = len(buf)
+        self._size = len(self._q)
         self._wi = 0
         self._ri = 0
         self._evput = asyncio.Event()  # Triggered by put, tested by get