Fix a segfault in pt/protocol test

Now that the pt code logs mp->argv[0] all over the place, we need to
be sure to set up mp->argv in our tests.

Bugfix on e603692adcd, not in any released version.

src/test/test_pt.c

@@ -94,6 +94,8 @@ test_pt_protocol(void)
   managed_proxy_t *mp = tor_malloc_zero(sizeof(managed_proxy_t));
   mp->conf_state = PT_PROTO_LAUNCHED;
   mp->transports = smartlist_new();
+  mp->argv = tor_malloc_zero(sizeof(char*)*2);
+  mp->argv[0] = tor_strdup("<testcase>");
 
   /* various wrong protocol runs: */