bpo-32710: Fix leak in Overlapped_WSASend() (GH-11469)

Fix a memory leak in asyncio in the ProactorEventLoop when ReadFile()
or WSASend() overlapped operation fail immediately: release the
internal buffer.

Misc/NEWS.d/next/Library/2019-01-08-14-00-52.bpo-32710.Sn5Ujj.rst

+Fix a memory leak in asyncio in the ProactorEventLoop when ``ReadFile()`` or
+``WSASend()`` overlapped operation fail immediately: release the internal
+buffer.

Modules/overlapped.c

@@ -723,6 +723,7 @@ do_ReadFile(OverlappedObject *self, HANDLE handle,
         case ERROR_IO_PENDING:
             Py_RETURN_NONE;
         default:
+            PyBuffer_Release(&self->user_buffer);
             self->type = TYPE_NOT_STARTED;
             return SetFromWindowsErr(err);
     }
@@ -1011,6 +1012,7 @@ Overlapped_WSASend(OverlappedObject *self, PyObject *args)
         case ERROR_IO_PENDING:
             Py_RETURN_NONE;
         default:
+            PyBuffer_Release(&self->user_buffer);
             self->type = TYPE_NOT_STARTED;
             return SetFromWindowsErr(err);
     }