Fix some memory errors in the recent coverity fixes.

Found by asan on travis :/

src/test/test_channelpadding.c

@@ -677,6 +677,7 @@ test_channelpadding_consensus(void *arg)
   memcpy(relay->identity_digest,
           ((channel_tls_t *)chan)->conn->identity_digest, DIGEST_LEN);
   smartlist_add(current_md_consensus->routerstatus_list, relay);
+  relay = NULL; /* Prevent double-free */
 
   tried_to_write_cell = 0;
   decision = channelpadding_decide_to_pad_channel(chan);

src/test/test_hs_descriptor.c

@@ -207,6 +207,7 @@ test_link_specifier(void *arg)
     tt_int_op(link_specifier_get_un_ipv4_port(ls), ==, spec.u.ap.port);
 
     link_specifier_free(ls);
+    ls = NULL;
     tor_free(b64);
   }
 
@@ -236,6 +237,7 @@ test_link_specifier(void *arg)
     tt_int_op(link_specifier_get_un_ipv6_port(ls), ==, spec.u.ap.port);
 
     link_specifier_free(ls);
+    ls = NULL;
     tor_free(b64);
   }
 
@@ -263,6 +265,7 @@ test_link_specifier(void *arg)
     tt_mem_op(spec.u.legacy_id, OP_EQ, id, DIGEST_LEN);
 
     link_specifier_free(ls);
+    ls = NULL;
     tor_free(b64);
   }