Fix crasher on 404 .pagespeed. resources w/a custom location

- Fix crasher on .pagespeed. resources that return 404
- Additionally, check for a wiped request context and make sure
  we do not dereference a null pointer, which is what hurt in
  the flow we hit above.
- Also, do not check fail when we would receive a stale event
  originating from a NgxBaseFetch that is no longer associated
  with our request context.

This is not the final change yet, as I want to back this with
tests where possible, and log warning when appropriate.

For #1081

src/ngx_base_fetch.cc

@@ -140,7 +140,30 @@ void NgxBaseFetch::ReadCallback(const ps_event_data& data) {
   }
   ps_request_ctx_t* ctx = ps_get_request_context(r);
 
-  CHECK(data.sender == ctx->base_fetch);
+  // If someone else wiped our request context, skip this event
+  // https://github.com/pagespeed/ngx_pagespeed/issues/1081
+  if (ctx == NULL) {
+      // TODO(oschaaf): log warning
+      return;
+  }
+
+  DCHECK(data.sender == ctx->base_fetch);
+  if (data.sender != ctx->base_fetch) {
+      // If someone else changed our request context, skip this event.
+      // Changing of request contexts may occur when nginx performs internal
+      // redirects, at which point we stil may have in-flight events.
+      // TODO(oschaaf): maintain a single request context associated to the
+      // request. Doing so also allows us to avoid doing the IPRO lookup 
+      // multiple times, and makes things a fraction less complex.
+      // When we fix that, we can turn the DCHECK above into a full CHECK 
+      // again.
+      // NOTE: Currently I have not been able to reproduce this actually 
+      // causing trouble, but there may be corner cases I missed.
+
+      // TODO(oschaaf): log warning(?)
+      return;
+  }
+
   CHECK(r->count > 0) << "r->count: " << r->count;
 
   int rc;

src/ngx_pagespeed.cc

@@ -284,7 +284,8 @@ ngx_int_t ps_base_fetch_handler(ngx_http_request_t* r) {
     // modules running after us to manipulate those responses.
     if (!status_ok && (ctx->base_fetch->base_fetch_type() != kHtmlTransform
                        && ctx->base_fetch->base_fetch_type() != kIproLookup)) {
-      return status_code;
+      ps_release_base_fetch(ctx);
+      return ngx_http_filter_finalize_request(r, NULL, status_code);
     }
 
     if (ctx->preserve_caching_headers != kDontPreserveHeaders) {