multi: skip IPv4 logic in multi_select_virtual_addr() if no pool is configured

multi: skip IPv4 logic in multi_select_virtual_addr() if no pool is configured

When no IPv4 pool is configured (but we have an IPv6 pool
only), the multi_select_virtual_addr() function will spit
a warning when allocating an address for a new client.
This happens because the code will check for some IPv4
bits and will see that they are missing.

However, these bits are not really important, because in
this use case we don't want to configure any IPv4 address
at all.

For this reason it is safe to wrap this entire logic in
an if-block that just does not execute when no IPv4 pool
is configured.

This avoids the warning and will also avoid any other
hidden side effect.

Reported-by: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200610084549.4028-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20012.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>