net: fix UB in VM_CL_getstati() and when sending the `items` stat

net: fix UB in VM_CL_getstati() and when sending the `items` stat

Simplifies the shifting in VM_CL_getstati() to avoid signed integer
overflow when the high bits are read with the arguments 23 and 9 (as
done in "quake15" and "ad" mods).

Refactors VM_CL_getstati() slightly, returns 0 when the index is out of
range (instead of whatever happened to be in that memory).

Masks off the bits that will overflow when packing the `items` stat for
sending (otherwise corruption of the low bits could occur on x86).

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>