patch 8.1.1612: cannot show an existing buffer in a popup window

Problem:    Cannot show an existing buffer in a popup window.
Solution:   Support buffer number argument in popup_create().
diff --git a/src/buffer.c b/src/buffer.c
index ee68bc9..74e0ea6 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -122,6 +122,23 @@
 }
 
 /*
+ * Ensure buffer "buf" is loaded.  Does not trigger the swap-exists action.
+ */
+    void
+buffer_ensure_loaded(buf_T *buf)
+{
+    if (buf->b_ml.ml_mfp == NULL)
+    {
+	aco_save_T	aco;
+
+	aucmd_prepbuf(&aco, buf);
+	swap_exists_action = SEA_NONE;
+	open_buffer(FALSE, NULL, 0);
+	aucmd_restbuf(&aco);
+    }
+}
+
+/*
  * Open current buffer, that is: open the memfile and read the file into
  * memory.
  * Return FAIL for failure, OK otherwise.