bpo-29587: allow chaining NULL exceptions in _gen_throw() (GH-19877)

This is a follow-up to GH-19823 that removes the check that the
exception value isn't NULL, prior to calling _PyErr_ChainExceptions().
This enables implicit exception chaining for gen.throw() in more
circumstances.

The commit also adds a test that a particular code snippet involving
gen.throw() doesn't crash.  The test shows why the new
`gi_exc_state.exc_type != Py_None` check that was added is necessary.
Without the new check, the code snippet (as well as a number of other
tests) crashes on certain platforms (e.g. Fedora but not Mac).

Lib/test/test_generators.py

@@ -332,6 +332,26 @@ class GeneratorThrowTest(unittest.TestCase):
         context = cm.exception.__context__
         self.assertEqual((type(context), context.args), (KeyError, ('a',)))
 
+    def test_throw_after_none_exc_type(self):
+        def g():
+            try:
+                raise KeyError
+            except KeyError:
+                pass
+
+            try:
+                yield
+            except Exception:
+                # Without the `gi_exc_state.exc_type != Py_None` in
+                # _gen_throw(), this line was causing a crash ("Segmentation
+                # fault (core dumped)") on e.g. Fedora 32.
+                raise RuntimeError
+
+        gen = g()
+        gen.send(None)
+        with self.assertRaises(RuntimeError) as cm:
+            gen.throw(ValueError)
+
 
 class YieldFromTests(unittest.TestCase):
     def test_generator_gi_yieldfrom(self):