Stop spurious clang shallow analysis null pointer errors

Avoid 4 null pointer errors under clang shallow analysis (the default when
building under Xcode) by using tor_assert() to prove that the pointers
aren't null. Resolves issue 13284 via minor code refactoring.
Stop test & bench build failures with --disable-curve25519

Ensure test & bench code that references curve25519 is disabled by the
appropriate macros. tor now builds with and without --disable-curve25519.
Merge branch 'master' of https://git.torproject.org/tor
Merge branch 'issue-13284-spurious-clang-analyze-shallow'