fix: reject IPMI packets with invalid CBC padding instead of crashing

cbc_pad in decrypt mode reads the last byte as the pad count, then
calls splice(@block, 0 - $count). If decrypted data is corrupt, the
pad count can exceed the array size, crashing with "Modification of
non-creatable array value attempted, subscript -16".

Return empty string on invalid padding so the caller treats it as a
decryption failure rather than accepting corrupted data as a valid
IPMI response.

Ref: #7511

xCAT-server/lib/perl/xCAT/IPMI.pm

@@ -763,6 +763,9 @@ sub cbc_pad {
         unless ($count) {
             return pack("C*", @block);
         }
+        if ($count > scalar @block) {
+            return "";
+        }
         splice @block, 0 - $count;
         return pack("C*", @block);
     }