(backport 21.1) Xnest: cursor: fix potentially uninitialized memory

It's safer to zero-out the cursor-private memory on allocation, instead of relying on being cleared initialized somewhere later.

Fixes: 3f3ff971 - Replace X-allocation functions with their C89 counterparts

Backport-Of: !1652 (merged)

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net