Fix Python mode error caused by incorrect indentation

* lisp/progmodes/python.el (python-indent--calculate-indentation):
Guard against negative indentation.  (Bug #65870)

* test/lisp/progmodes/python-tests.el
(python-indent-badly-indented-block-end): New test.

lisp/progmodes/python.el

@@ -1819,7 +1819,7 @@ possibilities can be narrowed to specific indentation points."
         (`(:after-block-end . ,start)
          ;; Subtract one indentation level.
          (goto-char start)
-         (- (current-indentation) python-indent-offset))
+         (max 0 (- (current-indentation) python-indent-offset)))
         (`(:at-dedenter-block-start . ,_)
          ;; List all possible indentation levels from opening blocks.
          (let ((opening-block-start-points

test/lisp/progmodes/python-tests.el

@@ -2149,6 +2149,15 @@ def test_re(string):
    (python-tests-look-at "else:")
    (should (= (python-indent-calculate-indentation) 4))))
 
+(ert-deftest python-indent-badly-indented-block-end ()
+  "Test BUG 65870 regression."
+  (python-tests-with-temp-buffer
+   "
+return
+"
+   (goto-char (point-max))
+   (should (= (python-indent-calculate-indentation) 0))))
+
 
 ;;; Filling