bpo-40559: Add Py_DECREF to _asynciomodule.c:task_step_impl() (GH-19990)

This fixes a possible memory leak in the C implementation of
asyncio.Task.

Author: cjerdonek

Misc/NEWS.d/next/Library/2020-05-05-08-12-51.bpo-40559.112wwa.rst

@@ -0,0 +1 @@
+Fix possible memory leak in the C implementation of :class:`asyncio.Task`.

Modules/_asynciomodule.c

@@ -2638,6 +2638,10 @@ task_step_impl(TaskObj *task, PyObject *exc)
     coro = task->task_coro;
     if (coro == NULL) {
         PyErr_SetString(PyExc_RuntimeError, "uninitialized Task object");
+        if (clear_exc) {
+            /* We created 'exc' during this call */
+            Py_DECREF(exc);
+        }
         return NULL;
     }