patch 8.1.0230: directly checking 'buftype' value

Problem:    Directly checking 'buftype' value.
Solution:   Add the bt_normal() function. (Yegappan Lakshmanan)

src/buffer.c

@@ -5626,6 +5626,15 @@ write_viminfo_bufferlist(FILE *fp)
 }
 #endif
 
+/*
+ * Return TRUE if "buf" is a normal buffer, 'buftype' is empty.
+ */
+    int
+bt_normal(buf_T *buf)
+{
+    return buf != NULL && buf->b_p_bt[0] == NUL;
+}
+
 /*
  * Return TRUE if "buf" is the quickfix buffer.
  */