net/http/fcgi: fix race in child.serve connection read

Guards the connection read with a mutex, because typeStdin
asynchronously and concurrently writes to the underlying conn.

Fixes #41167

Change-Id: Ia2610f4fde0bd4b108c54164095ea293980b0301
Reviewed-on: https://go-review.googlesource.com/c/go/+/252417


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>