internal/core/adt: fix nil pointer dereference

We had a nil check on envYield.node, but we accessed a node field
right before that nil check for apparently no good reason.

Fixes #2550.

Closes #2551 as merged as of commit f2dc179.

Signed-off-by: Artem V. Navrotskiy <bozaro@yandex.ru>
Change-Id: I0206d02d8c32ee82bc71e0c9b686965ed4f41399
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1167596
Reviewed-by: Paul Jolly <paul@myitcv.io>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/testdata/eval/issue2550.txtar

@@ -0,0 +1,40 @@
+-- in.cue --
+foo: string
+bar: close({})
+
+if bar.missing == "x" {
+	let _bar = bar
+	foo: _bar
+}
+-- out/compile --
+--- in.cue
+{
+  foo: string
+  bar: close({})
+  if (〈0;bar〉.missing == "x") {
+    let _bar#1 = 〈1;bar〉
+    foo: 〈0;let _bar#1〉
+  }
+}
+-- out/eval/stats --
+Leaks:  1
+Freed:  4
+Reused: 1
+Allocs: 4
+Retain: 5
+
+Unifications: 5
+Conjuncts:    5
+Disjuncts:    9
+-- out/eval --
+(_|_){
+  // [incomplete] undefined field: missing:
+  //     ./in.cue:4:8
+  foo: (_|_){
+    // [incomplete] undefined field: missing:
+    //     ./in.cue:4:8
+  }
+  bar: (#struct){
+  }
+  let _bar#1 = (_){ _ }
+}

internal/core/adt/comprehension.go

@@ -397,10 +397,10 @@ func (n *nodeContext) processComprehension(d *envYield, state vertexStatus) *Bot
 			}
 
 			// continue to collect other errors.
-			d.node.state.addBottom(err)
 			d.done = true
 			d.inserted = true
 			if d.node != nil {
+				d.node.state.addBottom(err)
 				ctx.PopArc(d.node)
 			}
 			return nil