Disable warning when reading CSRs from stdin.

[ckujau]

Coming across the same warning that was reported in PR#929 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