That might be a leak or it might not be a leak. If you were to add the backtrace of the allocation, that would be helpful.
More likely than not, it isn't a leak, but some bookkeeping information that is being stashed away by dyld that leaks/Instruments loses track of.
Given that it is 128 bytes and I'm guessing there is only one of them, I would encourage you to file a bug via http://bugreport.apple.com and then forget about it for now as, in the unlikely case that it really is a leak, you will be equally as unlikely to be able to fix it from within your application.
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