Fix segfault case

Lib/test/datetimetester.py

@@ -3095,6 +3095,7 @@ def test_fromisoformat_datetime_examples(self):
              self.theclass(2025, 1, 2, 3, 4, 5, 678901)),
             ('20250102T030405.6789010',
              self.theclass(2025, 1, 2, 3, 4, 5, 678901)),
+            ('2022W01', self.theclass(2022, 1, 3)),
             ('2022W52520', self.theclass(2022, 12, 26, 20, 0)),
             ('2022W527520', self.theclass(2023, 1, 1, 20, 0)),
             ('2026W01516', self.theclass(2025, 12, 29, 16, 0)),

Modules/_datetimemodule.c

@@ -5352,7 +5352,9 @@ _find_isoformat_datetime_separator(const char *dtstr, Py_ssize_t len) {
     static const char date_separator = '-';
     static const char week_indicator = 'W';
 
-    assert(len > 7);
+    if (len == 7) {
+        return 7;
+    }
 
     if (dtstr[4] == date_separator) {
         // YYYY-???