get_bandwidt_limits: ignore 'undef' as storage

If one of the storages passed in $storage_list was not defined
get_bandwidth_limit died (see [0], of an occurence of this).
This patch changes the behavior to ignore undef as storage instead.

[0] https://pve.proxmox.com/pipermail/pve-devel/2019-April/036515.html

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

PVE/Storage.pm

@@ -1646,6 +1646,7 @@ sub get_bandwidth_limit {
 
 	my %done;
 	foreach my $storage (@$storage_list) {
+	    next if !defined($storage);
 	    # Avoid duplicate checks:
 	    next if $done{$storage};
 	    $done{$storage} = 1;