SSI: return NGX_ERROR when timefmt memory allocation failed.

Previously, when using NGX_HTTP_SSI_ERROR, error was ignored in ssi processing,
thus timefmt could be accessed later in ngx_http_ssi_date_gmt_local_variable()
as part of "set" handler, or NULL format pointer could be passed to strftime().

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 6fb1fbe..b92ad4c 100644
--- a/src/http/modules/ngx_http_ssi_filter_module.c
+++ b/src/http/modules/ngx_http_ssi_filter_module.c
@@ -2388,7 +2388,7 @@
         ctx->timefmt.len = value->len;
         ctx->timefmt.data = ngx_pnalloc(r->pool, value->len + 1);
         if (ctx->timefmt.data == NULL) {
-            return NGX_HTTP_SSI_ERROR;
+            return NGX_ERROR;
         }
 
         ngx_cpystrn(ctx->timefmt.data, value->data, value->len + 1);