Check an index in ConstraintMatrix::add_entry.
Check an index in ConstraintMatrix::add_entry.

Without this check one can get segmentation faults in debug mode if the
current line is not present.

danshapero/dealii,naliboff/dealii,EGP-CIG-REU/dealii,kalj/dealii,EGP-CIG-REU/dealii,sairajat/dealii,naliboff/dealii,danshapero/dealii,EGP-CIG-REU/dealii,kalj/dealii,sairajat/dealii,danshapero/dealii,danshapero/dealii,danshapero/dealii,kalj/dealii,naliboff/dealii,danshapero/dealii,EGP-CIG-REU/dealii,naliboff/dealii,sairajat/dealii,EGP-CIG-REU/dealii,sairajat/dealii,sairajat/dealii,kalj/dealii,naliboff/dealii,EGP-CIG-REU/dealii,kalj/dealii,naliboff/dealii,EGP-CIG-REU/dealii,sairajat/dealii,naliboff/dealii,sairajat/dealii,kalj/dealii,danshapero/dealii,kalj/dealii