Fix crash when running androidmk against frameworks/base/Android.mk

The was a variable on the right-hand side of the assignment to
LOCAL_MODULE_CLASS , which wants a non-nil scope

Bug: 64266643
Test: androidmk frameworks/base/Android.mk
Test: androidmk prebuilts/misc/common/ddmlib/Android.mk

Change-Id: I52d33f5e5cb1179f84d4df149ef804268d67f7fb

androidmk/cmd/androidmk/android.go

@@ -449,7 +449,7 @@ func sanitize(sub string) func(ctx variableAssignmentContext) error {
 }
 
 func prebuiltClass(ctx variableAssignmentContext) error {
-	class := ctx.mkvalue.Value(nil)
+	class := ctx.mkvalue.Value(ctx.file.scope)
 	if v, ok := prebuiltTypes[class]; ok {
 		ctx.file.scope.Set("BUILD_PREBUILT", v)
 	} else {

androidmk/parser/scope.go

@@ -1,6 +1,8 @@
 package parser
 
-import "strings"
+import (
+	"strings"
+)
 
 type Scope interface {
 	Get(name string) string
@@ -84,6 +86,9 @@ func (v Variable) Value(scope Scope) string {
 	if ret, ok := v.EvalFunction(scope); ok {
 		return ret
 	}
+	if scope == nil {
+		panic("Cannot take the value of a variable in a nil scope")
+	}
 	return scope.Get(v.Name.Value(scope))
 }