dix: Check for NULL spriteInfo in GetPairedDevice

There is a race when reseting the XServer that causes spriteInfo to be
NULL in GetPairedDevice resulting a segfault and subsequent crash. The
problem was noticed when opening a connection, creating master devices,
destroying master devices and closing the connection during testing.

Signed-off-by: Arthur Williams <taaparthur@gmail.com>
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passed #703853
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