General Dirichlet boundary conditions can use unreasonable values for undefined DOFs

[saroberts]

This problem arises when applying the GD_LINEAR boundary condition to an internal, double-sided sideset where a DOF (a species concentration in this case) is only defined on one side of the sideset. When looping through the faces on the side that has the DOF properly defined, everything works fine. However, when later looping through the faces attached to the opposing block, an incorrect value is used. I did not verify, but it appears that it is either overflowing memory or, more likely, indexing back into some other set of variables.

The correct action in this case is to use a single-sided sideset, which we are now doing. However, I would expect that when a double-sided sideset is used and invalid variables are called, an error should be thrown.