Fix memory access violation on fatal error with Windows

Author: dlaugt

Python/pylifecycle.c

@@ -3079,6 +3079,8 @@ _Py_FatalError_PrintExc(PyThreadState *tstate)
 static void
 fatal_output_debug(const char *msg)
 {
+    assert(msg != NULL);
+
     /* buffer of 256 bytes allocated on the stack */
     WCHAR buffer[256 / sizeof(WCHAR)];
     size_t buflen = Py_ARRAY_LENGTH(buffer) - 1;
@@ -3360,7 +3362,12 @@ fatal_error(int fd, int header, const char *prefix, const char *msg,
     }
 
 #ifdef MS_WINDOWS
-    fatal_output_debug(msg);
+    if (msg) {
+        fatal_output_debug(msg);
+    }
+    else {
+        fatal_output_debug("<message not set>");
+    }
 #endif /* MS_WINDOWS */
 
     fatal_error_exit(status);