SSI: return NGX_ERROR when timefmt memory allocation failed.
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().

hy0kl/nginx,hy0kl/nginx,hy0kl/nginx