`location_invariant` should not be at `switch` `case` label

[sim642]

As discovered by @jprotopopov-ut, Goblint can output location_invariant at a switch case label.
This is not allowed by the YAML witness format: it's not be first character of a statement.

Also, inserting an assertion there would not mean the right thing: the assertion would be the last thing in the previous case.
Internally, these probably come from internal if branching nodes from a CIL-converted switch.
Not sure if we have enough information available in Goblint to distinguish the original source of the branchings.