mk-ca-bundle.pl: avoid warnings with -d without parameter

$Getopt::Std::STANDARD_HELP_VERSION = 1;

getopts('bd:fhilnqtuvw:');

if(!defined($opt_d)) {

    # to make plain "-d" use not cause warnings, and actually still work

    $opt_d = 'release';

}

# Use predefined URL or else custom URL specified on command line.

my $url = ( defined( $urls{$opt_d} ) ) ? $urls{$opt_d} : $opt_d;