Unlock KisDocument in case of an assert after saving

Adds unlocking of savingMutex to assert recover branches in
KisDocument::slotChildCompletedSavingInBackground.

libs/ui/KisDocument.cpp

@@ -996,24 +996,30 @@ bool KisDocument::initiateSavingInBackground(const QString actionName,
 
 void KisDocument::slotChildCompletedSavingInBackground(KisImportExportErrorCode status, const QString &errorMessage)
 {
-    KIS_SAFE_ASSERT_RECOVER(!d->savingMutex.tryLock()) {
+    KIS_SAFE_ASSERT_RECOVER_RETURN(isSaving());
+
+    KIS_SAFE_ASSERT_RECOVER(d->backgroundSaveDocument) {
         d->savingMutex.unlock();
         return;
     }
 
-    KIS_SAFE_ASSERT_RECOVER_RETURN(d->backgroundSaveDocument);
-
     if (d->backgroundSaveJob.flags & KritaUtils::SaveInAutosaveMode) {
         d->backgroundSaveDocument->d->isAutosaving = false;
     }
 
     d->backgroundSaveDocument.take()->deleteLater();
+
+    KIS_SAFE_ASSERT_RECOVER(d->backgroundSaveJob.isValid()) {
         d->savingMutex.unlock();
+        return;
+    }
 
-    KIS_SAFE_ASSERT_RECOVER_RETURN(d->backgroundSaveJob.isValid());
     const KritaUtils::ExportFileJob job = d->backgroundSaveJob;
     d->backgroundSaveJob = KritaUtils::ExportFileJob();
 
+    // unlock at the very end
+    d->savingMutex.unlock();
+
     KisUsageLogger::log(QString("Completed saving %1 (mime: %2). Result: %3")
                         .arg(job.filePath)
                         .arg(QString::fromLatin1(job.mimeType))