MINOR: cache: Consider invalid Age values as stale

Do not store responses that have an invalid age header (non numerical,
negative ...).
diff --git a/src/cache.c b/src/cache.c
index e0560e5..df6bd33 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -897,6 +897,8 @@
 			if (unlikely(object->age > true_maxage))
 				goto out;
 		}
+		else
+			goto out;
 		http_remove_header(htx, &ctx);
 	}