Fix fallback after another fallback.

Avoid decrementing the prefix len if already 0.

(cherry picked from commit 54438e4)

modules/drouting/drouting.c

@@ -2970,7 +2970,7 @@ static int do_routing(struct sip_msg* msg, dr_part_group_t * part_group,
 				void *data;
 				/* all rules under current prefix used -> reduce the prefix */
 				data = (void*)&avp_prefix->data;
-				((str*)data)->len = prefix_len-1;
+				((str*)data)->len = prefix_len?prefix_len-1:0;
 			}
 			LM_DBG("updating to %d, prefix %.*s \n",rule_idx,
 					prefix_len-(rule_idx?1:0),username.s);