core: tidy up Stream.on_receive() branches.

mitogen/core.py

@@ -741,16 +741,17 @@ class Stream(BasicStream):
         _vv and IOLOG.debug('%r.on_receive()', self)
 
         buf = self.receive_side.read()
-        if buf:
+        if not buf:
+            return self.on_disconnect(broker)
+
         if self._input_buf and self._input_buf_len < 128:
             self._input_buf[0] += buf
         else:
             self._input_buf.append(buf)
+
         self._input_buf_len += len(buf)
         while self._receive_one(broker):
             pass
-        else:
-            return self.on_disconnect(broker)
 
     HEADER_FMT = '>hhhLLL'
     HEADER_LEN = struct.calcsize(HEADER_FMT)