Fix crash using Sprite:close() Lua API on modified sprites (fix #2142)

src/app/crash/backup_observer.cpp

@@ -98,10 +98,16 @@ void BackupObserver::onRemoveDocument(Doc* doc)
     base::remove_from_container(m_documents, doc);
   }
   if (m_config->keepEditedSpriteDataFor > 0 &&
-      doc->needsBackup())
+      doc->needsBackup() &&
+      // If the backup is disabled, we don't need it (e.g. when the
+      // document is destroyed from a script with Sprite:close(), the
+      // backup is disabled)
+      !doc->inhibitBackup()) {
     m_closedDocs.push_back(doc);
-  else
+  }
+  else {
     m_session->removeDocument(doc);
+  }
 }
 
 void BackupObserver::backgroundThread()

src/app/doc_access.h

@@ -184,6 +184,12 @@ namespace app {
     void destroyDocument() {
       ASSERT(m_doc != nullptr);
 
+      // Don't create a backup for destroyed documents (e.g. documents
+      // are destroyed when they are used internally by Aseprite or by
+      // a script and then closed with Sprite:close())
+      if (m_doc->needsBackup())
+        m_doc->setInhibitBackup(true);
+
       m_doc->close();
       Doc* doc = m_doc;
       unlock();