os: Ignore dying client in ResetCurrentRequest

You might as well, it's harmless. Better, some cleanup code (like DRI2
swap wait) needs to run both normally and at client exit, so it
simplifies the callers to not need to check first. See 4308f5d3 for a
similar example.

Props: @ajax (Adam Jackson)

Fixes: xorg/xserver#211

Signed-off-by: Daniel Llewellyn <diddledan@ubuntu.com>
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