base: Reset CircleBuf size on flush()

The flush() method in CircleBuf resets the state of the circular
buffer, but fails to set size to zero. This obviously confuses code
that tries to determine the amount of data in the buffer. Set the size
to zero on flush.

src/base/circlebuf.cc

@@ -67,6 +67,7 @@ CircleBuf::flush()
 {
     _start = 0;
     _stop = 0;
+    _size = 0;
     _rollover = false;
 }