nginx: Fix memleak in nix-etag patch

The original patch introduced a new "real" variable which gets populated
(and allocated) via ngx_realpath(). It's properly freed in error
conditions but it won't be freed if ngx_http_set_etag returns
successfully.

Adding another ngx_free() just before returning fixes that memory leak.

I also fixed a small indentation issue along the way.

Signed-off-by: aszlig <aszlig@nix.build>