lib.fifo: in FIFOInterface.read(), check readable on the right cycle.

nmigen/lib/fifo.py

@@ -72,9 +72,9 @@ def __init__(self, width, depth, fwft):
 
     def read(self):
         """Read method for simulation."""
-        assert (yield self.readable)
         yield self.re.eq(1)
         yield
+        assert (yield self.readable)
         value = (yield self.dout)
         yield self.re.eq(0)
         return value