[netcdfgroup] strlen calls in NC_finddim and NC_findvar

["Greg Sjaardema" <gdsjaar@xxxxxxxxxx>]

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