Bug in mcases

The following code produces IllegalStateException:
```
\import Equiv
\import Function
\import Logic
\import Logic.Meta
\import Meta

\record Foo {
| arr : Array (Fin 1) 2
| allDiff : isInj arr
}

\func testFoo : Equiv {Foo} {Empty}
=> \new QEquiv {
| f x => \case \elim x \with {
| (a :: b :: nil, q) => \case \elim a, \elim b, \elim q \with {
| 0, 0, q => contradiction (q {1} {0} idp)
}
}
| ret => absurd
| ret_f _ => mcases
| f_sec => {?}
}
```