fix segfault introduced in r3007

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 23cf1b6..e705da6 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2883,7 +2883,7 @@
 
 #if (NGX_HTTP_CACHE)
 
-    if (u->cacheable) {
+    if (u->cacheable && r->cache) {
         time_t  valid;
 
         ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,