don't div by 0 during path selection

ChangeLog

@@ -1,4 +1,4 @@
-Changes in version 0.2.2.7-alpha - 2009-12-??
+Changes in version 0.2.2.7-alpha - 2010-01-??
   o Major features (performance):
     - When choosing which cells to relay first, we can now favor circuits
       that have been quiet recently, so as to get lower latency for
@@ -67,6 +67,10 @@ Changes in version 0.2.2.7-alpha - 2009-12-??
     - Fix statistics on client numbers by country as seen by bridges that
       were broken in 0.2.2.1-alpha. Also switch to reporting full 24-hour
       intervals instead of variable 12-to-48-hour intervals.
+    - If we're in the pathological case where there's no exit bandwidth
+      but there is non-exit bandwidth, or no guard bandwidth but there
+      is non-guard bandwidth, don't crash during path selection. Bugfix
+      on 0.2.0.3-alpha.
 
   o Removed features:
     - Remove the HSAuthorityRecordStats option that version 0 hidden

src/or/routerlist.c

@@ -1698,12 +1698,12 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, bandwidth_weight_rule_t rule,
      * For detailed derivation of this formula, see
      *   http://archives.seul.org/or/dev/Jul-2007/msg00056.html
      */
-    if (rule == WEIGHT_FOR_EXIT)
+    if (rule == WEIGHT_FOR_EXIT || !total_exit_bw)
       exit_weight = 1.0;
     else
       exit_weight = 1.0 - all_bw/(3.0*exit_bw);
 
-    if (rule == WEIGHT_FOR_GUARD)
+    if (rule == WEIGHT_FOR_GUARD || !total_guard_bw)
       guard_weight = 1.0;
     else
       guard_weight = 1.0 - all_bw/(3.0*guard_bw);