Disable warning when reading CSRs from stdin.

Disable warning when reading CSRs from stdin.

Coming across the same warning that was reported in
[PR#929](https://github.com/dehydrated-io/dehydrated/pull/929 "Suppress
openssl warning about reading from stdin") this is my attempt to disable
this warning. Instead of discarding stderr in total (this can still be
useful), we just use the "-in" parameter as hinted in the warning:

 $ foo=$(cat req.csr)
 $ <<<${foo} openssl req -noout -verify > /dev/null; echo $?
 Warning: Will read cert request from stdin since no -in option is given
 0

 $ <<<${foo} openssl req -in - -noout -verify > /dev/null; echo $?
 0