dix: don't XWarpPointer through the last slave anymore (#38313)

dix: don't XWarpPointer through the last slave anymore (#38313)

This line was introduced pre-1.6 to fix Bug 19297. The effect of warping
through the VCP then was that if a device had custom valuator ranges, the
warp position would be wrong. The better device for this effect is the the
XTest device.

This fixes a server crash where the lastSlave is a pointer device without
valuators (Bug 38313#0).

And while we're at it, make sure the Xinerama code-path does the same.

X.Org Bug 38313 <http://bugs.freedesktop.org/show_bug.cgi?id=38313>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
(cherry picked from commit 2bfb802839688ecf328119c4c6979390fc60348d)