FIx undefined behavior in dirvote.c

 Fix a bug in the voting algorithm that could yield incorrect results
 when a non-naming authority declared too many flags. Fixes bug 9200;
 bugfix on 0.2.0.3-alpha.

Found by coverity scan.

changes/bug9200

@@ -0,0 +1,5 @@
+  o Major bugfixes:
+    - Fix a bug in the voting algorithm that could yield incorrect results
+      when a non-naming authority declared too many flags. Fixes bug 9200;
+      bugfix on 0.2.0.3-alpha.
+

src/or/dirvote.c

@@ -1727,7 +1727,8 @@ networkstatus_compute_consensus(smartlist_t *votes,
           if (rs->flags & (U64_LITERAL(1) << i))
             ++flag_counts[flag_map[v_sl_idx][i]];
         }
-        if (rs->flags & (U64_LITERAL(1) << named_flag[v_sl_idx])) {
+        if (named_flag[v_sl_idx] >= 0 &&
+            (rs->flags & (U64_LITERAL(1) << named_flag[v_sl_idx]))) {
           if (chosen_name && strcmp(chosen_name, rs->status.nickname)) {
             log_notice(LD_DIR, "Conflict on naming for router: %s vs %s",
                        chosen_name, rs->status.nickname);