xorg-server: information leak and denial of service

Olivier Fourdan from Red Hat has discovered a protocol handling issue in
the way the X server code base handles the XkbSetGeometry request.

The issue stems from the server trusting the client to send valid string
lengths in the request data. A malicious client with string lengths
exceeding the request length can cause the server to copy adjacent
memory data into the XKB structs. This data is then available to the
client via the XkbGetGeometry request.
The data length is at least up to 64k, it is possible to obtain more
data by chaining strings, each string length is then determined by
whatever happens to be in that 16-bit region of memory.

A similarly crafted request can likely cause the X server to crash.