Infinite loop when adjusting a time stamp backwards across a DST boundary.

[Lee Ayres]

When adjusting a time-stamp across the DST boundary it is possible to 
throw the function into an infinite loop.

For example, in US Central Time the following never returns:

(adjust-timestamp @2014-03-09T03:00:00.000000-05:00 (:offset :sec -1))

Attached is a patch that bails out before getting caught in the loop.
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