Check for non-zero length first in read loop (#2465)

Since both the Python 2 and Python 3 cases need to handle the case of
zero and non-zero length separately, pull this check outside of the
Python 2 or 3 version checks to simplify the code a bit.

distributed/comm/tcp.py

@@ -186,16 +186,15 @@ def read(self, deserializers=None):
 
             frames = []
             for length in lengths:
-                if PY3 and self._iostream_has_read_into:
-                    frame = bytearray(length)
-                    if length:
+                if length:
+                    if PY3 and self._iostream_has_read_into:
+                        frame = bytearray(length)
                         n = yield stream.read_into(frame)
                         assert n == length, (n, length)
-                else:
-                    if length:
-                        frame = yield stream.read_bytes(length)
                     else:
-                        frame = b''
+                        frame = yield stream.read_bytes(length)
+                else:
+                    frame = b''
                 frames.append(frame)
         except StreamClosedError as e:
             self.stream = None