add two new error codes: NC_ENULLSTART and NC_ENULLCOUNT

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1780 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
checking starts == NULL should be after checking num == 0

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1781 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
just some comments

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1782 c368ceb8-fd28-0410-a457-e46ee1d7cd4a