Fix crash on fallback from a prefix-less rule.

If a prefix-less (empty prefix) rule matched, do not attempt to do fallback by reducing the len of the prefix.

(cherry picked from commit 6e7495a)

modules/drouting/drouting.c

@@ -1906,7 +1906,9 @@ static int do_routing(struct sip_msg* msg, dr_group_t *drg, int flags,
 	/* we are done reading -> unref the data */
 	lock_stop_read( ref_lock );
 
-	if ( flags & DR_PARAM_RULE_FALLBACK ) {
+	/* prepare data for fallback  only if instructed so and if there is
+	 * place for fallback (more rules or shorter prefix are available) */
+	if ( flags & DR_PARAM_RULE_FALLBACK && (prefix_len!=0 || rule_idx!=0) ) {
 		if ( !(flags & DR_PARAM_INTERNAL_TRIGGERED) ) {
 			/* first time ? we need to save some date, to be able to
 			   do the rule fallback later in "next_gw" */