Fix bug found by "ca": looking up a non-existent stream for a v1 control connection would cause a segfault. (No backport needed since 0.1.0 had only v0 connections.)

svn:r5001

src/or/control.c

@@ -599,7 +599,7 @@ get_stream(const char *id)
   if (!ok)
     return NULL;
   conn = connection_get_by_global_id(n_id);
-  if (conn->type != CONN_TYPE_AP)
+  if (!conn || conn->type != CONN_TYPE_AP)
     return NULL;
   return conn;
 }