CLEANUP: resolvers: Use ha_free() in srvrq_resolution_error_cb()

Two occurrences to "free(A);A=NULL;" may be replaced by a call to ha_free()
in the srvrq_resolution_error_cb() function.
diff --git a/src/server.c b/src/server.c
index 809666f..ee5bbac 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3296,12 +3296,10 @@
 		if (s->srvrq == srvrq) {
 			resolv_unlink_resolution(s->resolv_requester, 1);
 			srvrq_update_srv_status(s, 1);
-			free(s->hostname);
-			free(s->hostname_dn);
-			s->hostname        = NULL;
-			s->hostname_dn     = NULL;
-			s->hostname_dn_len = 0;
 			memset(&s->addr, 0, sizeof(s->addr));
+			ha_free(&s->hostname);
+			ha_free(&s->hostname_dn);
+			s->hostname_dn_len = 0;
 			s->svc_port = 0;
 		}
 		HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock);