Fix a NaN problem

See the commit log.
If you apply this patch
```diff
diff --git a/tests/advection/main.f90 b/tests/advection/main.f90
index 4f4c7f0..e06bfe1 100644
--- a/tests/advection/main.f90
+++ b/tests/advection/main.f90
@@ -41,8 +41,8 @@ program advection1d
 
     ! Initialize solver
     call new(solver,clawdata)
-    solver%rp1_ptwise => rp1_advection_ptwise
-!     solver%rp1_vectorized => rp1_advection_vectorized
+!    solver%rp1_ptwise => rp1_advection_ptwise
+    solver%rp1_vectorized => rp1_advection_vectorized
     solver%rp_data = c_loc(rp_aux)
 
     ! Start output loop
```
Then I am not getting any `NaN` in the file `fort.q0010`.

@mandli, can you have a look?