internal/core/adt: revert recent change

Was not ready to submit this and it is
causing trouble.

https://review.gerrithub.io/c/cue-lang/cue/+/1202776

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I232180342c058ab70f526ba3ab45bc19cdab8b1d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205394
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

Author: mpvl

internal/core/adt/fields.go

@@ -463,20 +463,27 @@ func (cc *closeContext) assignConjunct(ctx *OpContext, root *closeContext, c Con
 
 	c.CloseInfo.cc = nil
 
-	group := arc.group
-	pos = len(*group)
+	var group ConjunctGroup
+	if arc.group != nil {
+		group = *arc.group
+	}
+	pos = len(group)
 
-	added = !check || !hasConjunct(*group, c)
+	added = !check || !hasConjunct(group, c)
 	if added {
 		c.CloseInfo.cc = arc
 
 		if c.CloseInfo.cc.src != arc.src {
 			panic("Inconsistent src")
 		}
 
-		*group = append(*group, c)
+		group = append(group, c)
+		if arc.group == nil {
+			arc.group = &group
+		} else {
+			*arc.group = group
+		}
 	}
-
 	return arc, pos, added
 }