Fix Segfault during finalization (#615)

* Fix Segfault during finalization

Basically icorporates changes introduced in #603

* Fix typo

src/pybuffer.jl

@@ -42,13 +42,13 @@ end
 Release the reference to buffer `o`
 N.b. As per https://docs.python.org/3/c-api/buffer.html#c.PyBuffer_Release,
 It is an error to call this function on a PyBuffer that was not obtained via
-the python c-api function `PyObject_GetBuffer(), unless o.obj is a PyPtr(C_NULL)`
+the python c-api function `PyObject_GetBuffer()`, unless o.obj is a PyPtr(C_NULL)
 """
 function pydecref(o::PyBuffer)
     # note that PyBuffer_Release sets o.obj to NULL, and
     # is a no-op if o.obj is already NULL
     # TODO change to `Ref{PyBuffer}` when 0.6 is dropped.
-    ccall(@pysym(:PyBuffer_Release), Cvoid, (Any,), o)
+    _finalized[] || ccall(@pysym(:PyBuffer_Release), Cvoid, (Any,), o)
     o
 end