Do not pass NULL to log(%s) in dir_server_new.

This bug had existed since 0.2.4.7-alpha, but now that we have
FallbackDirs by default, it actually matters.

Fixes bug 19947; bugfix on 0.2.4.7-alpha or maybe 0.2.8.1-alpha.

Rubiate wrote the patch; teor wrote the changes file.

changes/bug19947

@@ -0,0 +1,4 @@
+ o Minor bugfixes (fallback directories):
+    - Avoid logging a NULL string pointer when loading fallback directory information.
+      Fixes bug 19947; bugfix on 0.2.4.7-alpha and 0.2.8.1-alpha.
+      Report and patch by "rubiate".

src/or/routerlist.c

@@ -4337,10 +4337,10 @@ dir_server_new(int is_authority,
 
   if (nickname)
     tor_asprintf(&ent->description, "directory server \"%s\" at %s:%d",
-                 nickname, hostname, (int)dir_port);
+                 nickname, hostname_, (int)dir_port);
   else
     tor_asprintf(&ent->description, "directory server at %s:%d",
-                 hostname, (int)dir_port);
+                 hostname_, (int)dir_port);
 
   ent->fake_status.addr = ent->addr;
   tor_addr_copy(&ent->fake_status.ipv6_addr, &ent->ipv6_addr);