nginx: initialize 'fc' earlier when closing stream
nginx: initialize 'fc' earlier when closing stream

Somewhat interestingly, GCC only reports a warning for this when all
optimizations are disabled (-O0), while Clang always reports it.

Reported by @morf (https://github.com/cloudflare/quiche/commit/349a63f672d80320b1f6989d16d8bdae5d1fba8e#commitcomment-38326179).

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche