[facecentered][convexcorner] fix the local Index logic

What this MR does / why does DuMux need it:

I've come across an error while using the new face centered staggered scheme. For convex corner geometries, localIndexOutsideScvfWithSameIntegrationPoint(scvf) is called from the outsideScvfWithSameIntegrationPoint(scvf) in the fluxvariables. The logic doesn't seem to be fool proof, in particular for the scvf with the local index 10.

As 10 is an even number, the function returns an index of 13, which does not typically exist, outside of elements with 2+ boundary faces. This change should fix this for the local index of 10.

As a reference, here is a confusing quick drawing with all of the local scvf indexes.

Notes for the reviewer

I believe that this has not been identified yet because we do not have many new freeflow tests with convex corner geometries, especially where the scvf with local index 10 would be involved.

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