We might as well clean up the witness memory, when it is this easy.

Spotted by: Rasmus Villemoes

bin/varnishd/cache/cache_main.c

@@ -350,7 +350,7 @@ child_main(int sigmagic, size_t altstksz)
 
 	AZ(pthread_key_create(&req_key, NULL));
 	AZ(pthread_key_create(&bo_key, NULL));
-	AZ(pthread_key_create(&witness_key, NULL));
+	AZ(pthread_key_create(&witness_key, free));
 	AZ(pthread_key_create(&name_key, NULL));
 
 	THR_SetName("cache-main");