Stop spurious clang shallow analysis null pointer errors

When clang does shallow analysis (the default when building under Xcode), it finds 4 null pointer errors.

We resolve this by using tor_assert() to prove that the pointers aren't null.

I'll attach a patch to this ticket as soon as I've created the changes entry.

version: 2.6.0-alpha git: 4fcc8d4e55256b96de400ad8cb04be451471853c (with #13280 (moved)) 5190ec0bc4c22d7bab756e21db6e357ba07379c4 (origin)