Fix a memory error in test_shared_random

Bug not in any released Tor.

src/test/test_shared_random.c

@@ -780,6 +780,7 @@ test_sr_setup_commits(void)
   /* Now during REVEAL phase save commit D by restoring its reveal. */
   set_sr_phase(SR_PHASE_REVEAL);
   save_commit_to_state(place_holder);
+  place_holder = NULL;
   tt_str_op(commit_d->encoded_reveal, OP_EQ,
             "DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD");
   /* Go back to an empty encoded reveal value. */