restore a generator's caller's exception state both on yield and (last) return

This prevents generator exception state from leaking into the caller.

Closes #12475.

Lib/test/test_exceptions.py

@@ -566,6 +566,21 @@ class ExceptionTests(unittest.TestCase):
             del g
             self.assertEqual(sys.exc_info()[0], TypeError)
 
+    def test_generator_leaking2(self):
+        # See issue 12475.
+        def g():
+            yield
+        try:
+            raise RuntimeError
+        except RuntimeError:
+            it = g()
+            next(it)
+        try:
+            next(it)
+        except StopIteration:
+            pass
+        self.assertEqual(sys.exc_info(), (None, None, None))
+
     def test_generator_finalizing_and_exc_info(self):
         # See #7173
         def simple_gen():