Fix potential null pointer deref during dirvote

Found by using clang's analyzer.

changes/dirvote_null_deref

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Fix a potential null-pointer dereference while computing a consensus.
+      Bugfix on tor-0.2.0.3-alpha, found with the help of clang's analyzer.
+

src/or/dirvote.c

@@ -444,9 +444,9 @@ compute_routerstatus_consensus(smartlist_t *votes, int consensus_method,
     if (cur && !compare_vote_rs(cur, rs)) {
       ++cur_n;
     } else {
-      if (cur_n > most_n ||
-          (cur && cur_n == most_n &&
-           cur->status.published_on > most_published)) {
+      if (cur && (cur_n > most_n ||
+                  (cur_n == most_n &&
+                   cur->status.published_on > most_published))) {
         most = cur;
         most_n = cur_n;
         most_published = cur->status.published_on;