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/version.c

@@ -758,6 +758,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2018,
 /**/
     2017,
 /**/