bpo-38631: Avoid Py_FatalError() in _memory_release() (GH-18214)

If the export count is negative, _memory_release() now raises a
SystemError and returns -1, rather than calling Py_FatalError()
which aborts the process.

Objects/memoryobject.c

@@ -1048,7 +1048,8 @@ _memory_release(PyMemoryViewObject *self)
         return -1;
     }
 
-    Py_FatalError("_memory_release(): negative export count");
+    PyErr_SetString(PyExc_SystemError,
+                    "_memory_release(): negative export count");
     return -1;
 }