JSInterp: fix switch instruction with condition inside it's branches

[LeoVgr]

Fix null access when trying to use JSInterp while interpreting code that contains a switch case with a condition and no other instructions (see example below)

var t = 0;

// OK
switch {
    default:
        if (true)
            t = 1;
        t = 2;
}

// Crash
switch {
    default:
        if (true)
            t = 1;
}