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[netcdfgroup] strlen calls in NC_finddim and NC_findvar

I have a monstrous file with several thousand dimensions and variables which is running slower than it should. I investigated the runtime and found that strlen was the major time user in the NC_finddim and NC_findvar calls. The obvious optimization was to cache the length of the name instead of calling strlen each time. However, when I went to do this, I discovered that the length is already cached as the nchars field in the NC_string struct.
I did some checks in the code and also added some assertions to the code and verified that, as far as I can tell, nchars is the correct length of the string. Is there a reason that it isn't used and strlen() is called instead? Switching the code to use nchars dropped my execution time from 20 units to 6 units. I would like to make the switch, but wondered if there was some strange corner case where the nchars value is incorrect and will cause problems. 

Thanks,
--Greg
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