Clean up test_hs.c: warning fix; tor_free() usage.

My OSX laptop rightly gave a warning because of sticking strlen() into
an int, but once I took a closer look... it appears that the strlen()
was part of a needlessly verbose implementation for tor_strdup().

While I was there, I fixed the usage of tor_free() in test_hs.c: It
checks for NULL, and it zeros its argument.  So instead of
    if (foo) {
      tor_free(foo);
      foo = NULL;
    }
we should just say
    tor_free(foo);
test_config.c: log which address should have failed