Fix compiler bug: "return" without value causes assertion failure. (Thanks Xuetian Weng)
Fix compiler bug: "return" without value causes assertion failure.
(Thanks Xuetian Weng)

* dpy/compiler/pygen.py(PythonGenerator.visit_ReturnStmt): Guard
  against None.

mayli/DistAlgo,sghosh1991/distalgo,sghosh1991/distalgo,mayli/DistAlgo