fix segfault introduced in r2602 if there is 502/504 error
and proxy/fastcgi_cache is defined without proxy/fastcgi_cache_valid

src/http/ngx_http_file_cache.c

diff --git a/src/http/ngx_http_file_cache.c b/src/http/ngx_http_file_cache.c
index 78f4773..0c23e46 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -1290,6 +1290,10 @@
     ngx_uint_t               i;
     ngx_http_cache_valid_t  *valid;
 
+    if (cache_valid == NULL) {
+        return 0;
+    }
+
     valid = cache_valid->elts;
     for (i = 0; i < cache_valid->nelts; i++) {