patch 7.4.2018

Problem:    buf_valid() can be slow when there are many buffers.
Solution:   Add bufref_valid(), only go through the buffer list when a buffer
            was freed.

src/structs.h

@@ -69,6 +69,14 @@ typedef struct frame_S		frame_T;
 typedef int			scid_T;		/* script ID */
 typedef struct file_buffer	buf_T;  /* forward declaration */
 
+/* Reference to a buffer that stores the value of buf_free_count.
+ * bufref_valid() only needs to check "buf" when the count differs.
+ */
+typedef struct {
+    buf_T   *br_buf;
+    int	    br_buf_free_count;
+} bufref_T;
+
 /*
  * This is here because regexp.h needs pos_T and below regprog_T is used.
  */