Appease flexelint

cache/cache_vrt.c  916  Warning 438: Last value assigned to variable
'berr' (defined at line 849) not used

berr is intentionally NULLed to clarify that no dynamic value must stay
alive after the BAN_Abandon()

Ref 6eee0074

bin/varnishd/cache/cache_vrt.c

@@ -908,6 +908,7 @@ VRT_ban_string(VRT_CTX, VCL_STRING str)
 			err = "Unknown error (workspace overflow)";
 		berr = NULL;
 	}
+	AZ(berr);
 	if (bp != NULL)
 		BAN_Abandon(bp);
 	VAV_Free(av);