TLSProxy/Proxy.pm: handle -1 as return value from waitpid.

On rare occasion 's_server | perl -ne print' can complete before
corresponding waitpid, which on Windows can results in -1 return
value. This is not an error, don't treat it like one. Collect
even return value from s_server.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5907)