access control: avoid "uninitialized value" warning if using IP ranges

access control: avoid "uninitialized value" warning if using IP ranges

ALLOW_FROM/DENY_FROM accept any syntax understood by Net::IP. However,
if an IP range like "10.1.1.1-10.1.1.3" is configured, a confusing
Perl warning is printed to the syslog on a match:

  Use of uninitialized value in concatenation (.) or string at [...]

The reason is that we use Net::IP::prefix to prepare a debug message,
but this returns undef if a range was specified. To avoid the warning,
use Net::IP::print to obtain a string representation instead.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>