fix a seg fault on solaris

svn:r2313

src/common/tortls.c

@@ -294,7 +294,7 @@ tor_tls_context_new(crypto_pk_env_t *identity,
   char nn2[1024];
   int client_only;
   SSL_CTX **ctx;
-  sprintf(nn2, "%s <identity>", nickname);
+  sprintf(nn2, "%s <identity>", nickname ? nickname : "null");
 
   tor_tls_init();