Segfault on eval of case expression

[Jiten Kumar Pathy via Bird-users]

Hello,

The following filter causes bird2 to segfault:
````
filter test {
  case 2 {
      1: accept;
      else: reject;
  }
}
````
I have locally added `NEVER_CONSTANT` to `FI_SWITCH` instruction as a 
temporary workaround.