remote: check for negative array lengths before allocation

remote: check for negative array lengths before allocation

While the C API entry points will validate non-negative lengths
for various parameters, the RPC server de-serialization code
will need to allocate memory for arrays before entering the C
API. These allocations will thus happen before the non-negative
length check is performed.

Passing a negative length to the g_new0 function will usually
result in a crash due to the negative length being treated as
a huge positive number.

This was found and diagnosed by ALT Linux Team with AFLplusplus.

CVE-2024-2494
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Found-by: Alexandr Shashkin <dutyrok@altlinux.org>
Co-developed-by: Alexander Kuznetsov <kuznetsovam@altlinux.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>