x86: uv: xpc NULL deref when mesq becomes empty

Under heavy load conditions, our set of xpc messages may become exhausted.
 The code handles this correctly with the exception of the management code
which hits a NULL pointer dereference.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>