Proper NULL checking for hsdesc publication

Fix a dereference-then-NULL-check sequence. This bug wasn't triggered
in the wild, but we should fix it anyways in case it ever happens.
Also make sure users get a note about this being a bug when they
see it in their log.

Thanks to ekir for discovering and reporting this bug.

ChangeLog

@@ -2,6 +2,9 @@ Changes in version 0.2.2.10-alpha - 2010-??-??
   o Minor bugfixes:
     - Fix a memleak in the EXTENDCIRCUIT logic. Spotted by coverity.
       Bugfix on 0.2.2.9-alpha.
+    - Fix a dereference-then-NULL-check sequence when publishing
+      descriptors. Bugfix on tor-0.2.1.5-alpha. Discovered by ekir,
+      fixes bug 1255.
 
 Changes in version 0.2.2.9-alpha - 2010-02-22
   o Directory authority changes:

src/or/rendcommon.c

@@ -456,17 +456,17 @@ rend_encode_v2_descriptors(smartlist_t *descs_out,
   size_t ipos_len = 0, ipos_encrypted_len = 0;
   int k;
   uint32_t seconds_valid;
-  crypto_pk_env_t *service_key = auth_type == REND_STEALTH_AUTH ?
-                                 client_key : desc->pk;
+  crypto_pk_env_t *service_key;
+  if (!desc) {
+    log_warn(LD_BUG, "Could not encode v2 descriptor: No desc given.");
+    return -1;
+  }
+  service_key = (auth_type == REND_STEALTH_AUTH) ? client_key : desc->pk;
   tor_assert(service_key);
   if (auth_type == REND_STEALTH_AUTH) {
     descriptor_cookie = smartlist_get(client_cookies, 0);
     tor_assert(descriptor_cookie);
   }
-  if (!desc) {
-    log_warn(LD_REND, "Could not encode v2 descriptor: No desc given.");
-    return -1;
-  }
   /* Obtain service_id from public key. */
   crypto_pk_get_digest(service_key, service_id);
   /* Calculate current time-period. */