[3.6] bpo-33622: Fix issues with handling errors in the GC. (GH-7078) (GH-7095)

[3.6] bpo-33622: Fix issues with handling errors in the GC. (GH-7078) (GH-7095)

* Fixed a leak when the GC fails to add an object with __del__ into
  the gc.garbage list.
* PyGC_Collect() can now be called when an exception is set and
  preserves it.
* Fixed an undefined behavior with comparing a dead pointer with NULL.
(cherry picked from commit 301e3cc8a5bc68c5347ab6ac6f83428000d31ab2)