Add an assertion to compute_weighted_bandwidths()

This should please coverity, and fix CID 1415722.  It didn't
understand that networkstatus_get_param() always returns a value
between its minimum and maximum values.

src/feature/nodelist/node_select.c

@@ -585,6 +585,7 @@ compute_weighted_bandwidths(const smartlist_t *sl,
   }
 
   weight_scale = networkstatus_get_weight_scale_param(NULL);
+  tor_assert(weight_scale >= 1);
 
   if (rule == WEIGHT_FOR_GUARD) {
     Wg = networkstatus_get_bw_weight(NULL, "Wgg", -1);