When we see an incomplete read with 0 bytes, it's a disconnect

Partially fixes mitmproxy/mitmproxy:#593

netlib/tcp.py

@@ -225,6 +225,9 @@ class Reader(_FileLike):
         """
         result = self.read(length)
         if length != -1 and len(result) != length:
+            if not result:
+                raise NetLibDisconnect()
+            else:
                 raise NetLibIncomplete(
                     "Expected %s bytes, got %s" % (length, len(result))
                 )