fix segfault if cache key is larger than upstream buffer size
patch by Lanshun Zhou

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index f9f09cb..ad5b449 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -661,6 +661,15 @@
 
         ngx_http_file_cache_create_key(r);
 
+        if (r->cache->header_start >= u->conf->buffer_size) {
+            ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
+                "cache key too large, increase upstream buffer size %uz",
+                u->conf->buffer_size);
+
+            r->cache = NULL;
+            return NGX_DECLINED;
+        }
+
         switch (ngx_http_test_predicates(r, u->conf->cache_bypass)) {
 
         case NGX_ERROR: