Fix null pointer dereference printing an expression.

Parser.parseVariable method should always set the value of the variable
it creates. Failure to do so may end up in the following:

```
$ androidmk  <(printf "FOO:=(X)\nFOO:=bar\n")
parse error:
<input>:3:1: variable already set, previous assignment: FOO@<input>:1:5 = %!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference) (%!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference)) false
```

The cause is that calling Parser.Parse to parse `FOO=abc` created
a Variable instance with nil value, causing panic on print attempt.

Test: m androidmk && androidmk  <(printf "FOO:=(X)\nFOO:=bar\n")
(should print:
ERROR:  parse error:
<input>:3:1: variable already set, previous assignment: FOO@<input>:1:5 = X = Not Evaluated (X = Not Evaluated) false)

Change-Id: I296d7984df6d8796e0075f9eb692b234f8c94f08

parser/ast.go

@@ -164,6 +164,7 @@ const (
 	Int64Type
 	ListType
 	MapType
+	NotEvaluatedType
 )
 
 func (t Type) String() string {
@@ -178,6 +179,8 @@ func (t Type) String() string {
 		return "list"
 	case MapType:
 		return "map"
+	case NotEvaluatedType:
+		return "notevaluated"
 	default:
 		panic(fmt.Errorf("Unknown type %d", t))
 	}
@@ -476,6 +479,29 @@ func (c Comment) Text() string {
 	return string(buf)
 }
 
+type NotEvaluated struct {
+	Position scanner.Position
+}
+
+func (n NotEvaluated) Copy() Expression {
+	return NotEvaluated{Position: n.Position}
+}
+
+func (n NotEvaluated) String() string {
+	return "Not Evaluated"
+}
+
+func (n NotEvaluated) Type() Type {
+	return NotEvaluatedType
+}
+
+func (n NotEvaluated) Eval() Expression {
+	return NotEvaluated{Position: n.Position}
+}
+
+func (n NotEvaluated) Pos() scanner.Position { return n.Position }
+func (n NotEvaluated) End() scanner.Position { return n.Position }
+
 func endPos(pos scanner.Position, n int) scanner.Position {
 	pos.Offset += n
 	pos.Column += n

parser/parser.go

@@ -484,6 +484,8 @@ func (p *parser) parseVariable() Expression {
 				}
 				value = assignment.Value
 			}
+		} else {
+			value = &NotEvaluated{}
 		}
 		value = &Variable{
 			Name:    text,

parser/parser_test.go

@@ -1122,3 +1122,24 @@ func TestParserEndPos(t *testing.T) {
 		}
 	}
 }
+
+func TestParserNotEvaluated(t *testing.T) {
+	// When parsing without evaluation, create variables correctly
+	scope := NewScope(nil)
+	input := "FOO=abc\n"
+	_, errs := Parse("", bytes.NewBufferString(input), scope)
+	if errs != nil {
+		t.Errorf("unexpected errors:")
+		for _, err := range errs {
+			t.Errorf("  %s", err)
+		}
+		t.FailNow()
+	}
+	assignment, found := scope.Get("FOO")
+	if !found {
+		t.Fatalf("Expected to find FOO after parsing %s", input)
+	}
+	if s := assignment.String(); strings.Contains(s, "PANIC") {
+		t.Errorf("Attempt to print FOO returned %s", s)
+	}
+}