[qemu-server] fix #5040: qm: migrate: prevent perl error on missing mapped devices

[Elias Huhsovitz <e.huhsovitz@proxmox.com>]

When a VM configuration references a mapped device (USB, PCI, or
directory) that does not exist in the cluster's mapping configuration,
the `PVE::Mapping::*::get_node_mapping` functions return `undef`.

The migration attempted to dereference the result directly,
which caused an error:

  Can't use an undefined value as an ARRAY reference at .../Helpers.pm

Check if the returned entry is defined before attempting to dereference
it as an array. This allows the migration check to gracefully handle
missing mappings and report them to the user.

Signed-off-by: Elias Huhsovitz <e.huhsovitz@proxmox.com>
---
 src/PVE/QemuMigrate/Helpers.pm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PVE/QemuMigrate/Helpers.pm b/src/PVE/QemuMigrate/Helpers.pm
index 466517da..c0e54831 100644
--- a/src/PVE/QemuMigrate/Helpers.pm
+++ b/src/PVE/QemuMigrate/Helpers.pm
@@ -60,7 +60,7 @@ sub check_local_resources {
             } elsif ($type eq 'dir') {
                 $entry = PVE::Mapping::Dir::get_node_mapping($dir_map, $id, $node);
             }
-            if (!scalar($entry->@*)) {
+            if (!$entry || !scalar($entry->@*)) {
                 push @{ $missing_mappings_by_node->{$node} }, $key;
             }
         }
-- 
2.47.3