nf90_get_att returning character variables with c null terminator

[bena-nasa]

While trying to use a NetCDF4 file provided by a user I was hitting an error that we ultimately traced to the fact that nf90_get_att was returning a character string with a C null terminator at the end. We are not sure if it is this file but we do need to protect against this. I will make a PR.

[weiyuan-jiang]

I think we should use "pfio_get_att_string" instead of "nf90_get_att" . But I found many string version of nf90_get_att .