PySys_Audit() checks arguments

* Reject NULL event argument with ValueError.
* Reject "N" format in the format argument with ValueError.

Python/sysmodule.c

@@ -189,11 +189,6 @@ static int
 sys_audit_tstate(PyThreadState *ts, const char *event,
                  const char *argFormat, va_list vargs)
 {
-    /* N format is inappropriate, because you do not know
-       whether the reference is consumed by the call.
-       Assert rather than exception for perf reasons */
-    assert(!argFormat || !strchr(argFormat, 'N'));
-
     if (!ts) {
         /* Audit hooks cannot be called with a NULL thread state */
         return 0;
@@ -209,6 +204,21 @@ sys_audit_tstate(PyThreadState *ts, const char *event,
         return 0;
     }
 
+    if (event == NULL) {
+        _PyErr_SetString(ts, PyExc_ValueError,
+                         "event argument must not be NULL");
+        return -1;
+    }
+
+    /* N format is inappropriate, because you do not know
+       whether the reference is consumed by the call.
+       Assert rather than exception for perf reasons */
+    if (argFormat != NULL && strchr(argFormat, 'N')) {
+        _PyErr_SetString(ts, PyExc_ValueError,
+                         "N format must not be used in the format argument");
+        return -1;
+    }
+
     PyObject *eventName = NULL;
     PyObject *eventArgs = NULL;
     PyObject *hooks = NULL;