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 legacy/trac#13280 (moved)) 5190ec0b (origin)