test_doctest_id: avoid UnboundLocalError with internal errors

Author: blueyed

testing/acceptance_test.py

@@ -861,10 +861,12 @@ def test_doctest_id(self, testdir):
         )
         result = testdir.runpytest("-rf")
         lines = result.stdout.str().splitlines()
+        testid = None
         for line in lines:
             if line.startswith(("FAIL ", "FAILED ")):
                 _fail, _sep, testid = line.partition(" ")
                 break
+        assert testid is not None
         result = testdir.runpytest(testid, "-rf")
         result.stdout.fnmatch_lines(
             ["FAILED test_doctest_id.txt::test_doctest_id.txt", "*1 failed*"]