Remove invalid check in find_first_of

[Paolo Carlini]

Paolo Carlini wrote:

> Seriously, now I'm sleepy: will review the patch tomorrow, if nobody 
> beats me, indeed.

Ok, the patch looks definitely correct, I'm going to apply it. Thanks!

A recurring theme in this area is bringing over the latest concept_check 
code in Boost: what do you think? Certainly, there are also non-trivial 
infrastructural changes, not only bug fixes, but IMO we should seriously 
consider that option...

Paolo.