patch 8.1.1844: buffer no longer unloaded when adding text properties

Problem:    Buffer no longer unloaded when adding text properties to it.
Solution:   Do not create the memfile. (closes #4808)
diff --git a/src/textprop.c b/src/textprop.c
index a976414..309f3cb 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -256,7 +256,10 @@
     }
 
     if (buf->b_ml.ml_mfp == NULL)
-	ml_open(buf);
+    {
+	emsg(_("E275: Cannot add text property to unloaded buffer"));
+	return;
+    }
 
     for (lnum = start_lnum; lnum <= end_lnum; ++lnum)
     {