core: fix property getter method for NFileDescriptorStore bus property

core: fix property getter method for NFileDescriptorStore bus property

Since da6053d0a7c16795e7fac1f9ba6694863918a597 this is a size_t, not an
unsigned. The difference doesn't matter on LE archs, but it matters on
BE (i.e. s390x), since we'll return entirely nonsensical data.

Let's fix that.

Follow-up-for: da6053d0a7c16795e7fac1f9ba6694863918a597

An embarassing bug introduced in 2018... That made me scratch my head
for way too long, as it made #27135 fail on s390x while it passed
everywhere else.