fix segfault if empty stub block is used second time

src/http/modules/ngx_http_ssi_filter_module.c

diff --git a/src/http/modules/ngx_http_ssi_filter_module.c b/src/http/modules/ngx_http_ssi_filter_module.c
index 5e17cb5..1d7730d 100644
--- a/src/http/modules/ngx_http_ssi_filter_module.c
+++ b/src/http/modules/ngx_http_ssi_filter_module.c
@@ -1983,6 +1983,7 @@
 
         if (bl[i].count++) {
 
+            out = NULL;
             ll = &out;
 
             for (tl = bl[i].bufs; tl; tl = tl->next) {