Avoid calling SMARTLIST_FOREACH on a NULL smartlist in tests

Two of the test functions could possibly call SMARTLIST_FOREACH on a NULL smartlist.

This branch fixes the issue:

Repository: https://github.com/teor2345/tor.git Branch: avoid-NULL-smartlist-foreach

Check if each smartlist is NULL before calling SMARTLIST_FOREACH on it.

Bug discovered by the clang static analyzer. Apple clang 600.0.56 (LLVM 3.5svn) on x86_64-apple-darwin14.1.0.

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