SSI: handling of subrequests from other modules (ticket #1263). As the SSI parser always uses the context from the main request for storing variables and blocks, that context should always exist for subrequests using SSI, even though the main request does not necessarily have SSI enabled. However, `ngx_http_get_module_ctx(r->main, ...)` is getting NULL in such cases, resulting in the worker crashing SIGSEGV when accessing its attributes. This patch links the first initialized context to the main request, and upgrades it only when main context is initialized.