Upstream: ngx_http_upstream_store() error handling fixes.

Previously, ngx_http_map_uri_to_path() errors were not checked in
ngx_http_upstream_store().  Moreover, in case of errors temporary
files were not deleted, as u->store was set to 0, preventing cleanup
code in ngx_http_upstream_finalize_request() from removing them.  With
this patch, u->store is set to 0 only if there were no errors.

Reported by Feng Gu.