migration: Receiving a zero page non zero is an error

We don't allow non zero compressed pages since:

commit 3edcd7e6eb
Author: Peter Lieven <pl@kamp.de>
Date:   Tue Mar 26 10:58:35 2013 +0100

    migration: search for zero instead of dup pages

RDMA case is a bit more complicated, but they don't handle it since:

commit a1febc4950
Author: Richard Henderson <rth@twiddle.net>
Date:   Mon Aug 29 11:46:14 2016 -0700

    cutils: Export only buffer_is_zero

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231019085259.13307-2-quintela@redhat.com>

migration/rdma.c

@@ -3592,7 +3592,11 @@ int rdma_registration_handle(QEMUFile *f)
 
             host_addr = block->local_host_addr +
                             (comp->offset - block->offset);
-
+            if (comp->value) {
+                error_report("rdma: Zero page with non-zero (%d) value",
+                             comp->value);
+                goto err;
+            }
             ram_handle_compressed(host_addr, comp->value, comp->length);
             break;