cue: prevent crash in JSON conversion

Caught by new test set. Tests will come later.

Change-Id: I33daf892ac90c98284545386d0504f2c87f75f04
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5820
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/types.go

@@ -936,8 +936,9 @@ func (v Value) marshalJSON() (b []byte, err error) {
 		if len(st.comprehensions) > 0 {
 			// This should always evaluate to incomplete. However, fall back
 			// to a bad error message, rather than crashing, in case it doesn't.
-			err, _ := st.comprehensions[0].comp.evalPartial(ctx).(*bottom)
-			return nil, toMarshalErr(v, err)
+			if err, ok := st.comprehensions[0].comp.evalPartial(ctx).(*bottom); ok {
+				return nil, toMarshalErr(v, err)
+			}
 		}
 
 		if err != nil {