Fix memory leak in test_sr_setup_commits().

This patch fixes a memory leak in test_sr_setup_commits() where the
place_holder is allocated, but never freed again.

See: Coverity CID 1437440.

src/test/test_shared_random.c

@@ -788,6 +788,7 @@ test_sr_setup_commits(void)
   tt_assert(!commit_has_reveal_value(commit_d));
 
  done:
+  tor_free(place_holder);
   authority_cert_free(auth_cert);
 }