Issue #14173: Avoid crashing when reading a signal handler during interpreter shutdown.

Misc/NEWS

@@ -69,6 +69,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #14173: Avoid crashing when reading a signal handler during
+  interpreter shutdown.
+
 - Issue #15902: Fix imp.load_module() accepting None as a file when loading an
   extension module.
 

Modules/signalmodule.c

@@ -339,7 +339,10 @@ signal_signal(PyObject *self, PyObject *args)
     Handlers[sig_num].tripped = 0;
     Py_INCREF(obj);
     Handlers[sig_num].func = obj;
-    return old_handler;
+    if (old_handler != NULL)
+        return old_handler;
+    else
+        Py_RETURN_NONE;
 }
 
 PyDoc_STRVAR(signal_doc,
@@ -367,8 +370,13 @@ signal_getsignal(PyObject *self, PyObject *args)
         return NULL;
     }
     old_handler = Handlers[sig_num].func;
-    Py_INCREF(old_handler);
-    return old_handler;
+    if (old_handler != NULL) {
+        Py_INCREF(old_handler);
+        return old_handler;
+    }
+    else {
+        Py_RETURN_NONE;
+    }
 }
 
 PyDoc_STRVAR(getsignal_doc,