silence compiler warning

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1949 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
return error immediately when ncid is invalid

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1950 c368ceb8-fd28-0410-a457-e46ee1d7cd4a