patch 8.1.0230: directly checking 'buftype' value

Problem:    Directly checking 'buftype' value.
Solution:   Add the bt_normal() function. (Yegappan Lakshmanan)
diff --git a/src/fileio.c b/src/fileio.c
index 07b241e..281191c 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -6897,7 +6897,7 @@
      * this buffer. */
     if (buf->b_ffname == NULL
 	    || buf->b_ml.ml_mfp == NULL
-	    || *buf->b_p_bt != NUL
+	    || !bt_normal(buf)
 	    || buf->b_saving
 	    || busy
 #ifdef FEAT_NETBEANS_INTG