Fix clang address of struct member always non-NULL in SSL master key

clang complains that the address of struct member in an assert in
SSL_SESSION_get_master_key is always non-NULL.
Instead, check each pointer argument is non-NULL before using it.

Fix on f90a704f1258 from 27 May 2015, not in any released version of tor.

src/common/tortls.c

@@ -2631,10 +2631,11 @@ SSL_get_server_random(SSL *s, uint8_t *out, size_t len)
 static size_t
 SSL_SESSION_get_master_key(SSL_SESSION *s, uint8_t *out, size_t len)
 {
+  tor_assert(s);
   if (len == 0)
     return s->master_key_length;
   tor_assert(len == (size_t)s->master_key_length);
-  tor_assert(s->master_key);
+  tor_assert(out);
   memcpy(out, s->master_key, len);
   return len;
 }