Reordering of nodal Dirichlet conditions fails

MWE on master

```julia
using Ferrite
grid = generate_grid(Line, (1,));

ip = Lagrange{RefLine,1}()

dh = DofHandler(grid)
add!(dh, :u, ip)
close!(dh);

ch = ConstraintHandler(dh);
∂Ω = [1] # Triggers error in renumbering
# ∂Ω = getfacetset(grid, "left") # Works
dbc = Dirichlet(:u, ∂Ω, (x, t) -> 0)
add!(ch, dbc);
close!(ch)

perm = [2,1]
Ferrite.renumber!(dh,ch,perm)
```
(Edit by @KnutAM moving ip above dh)