patch 7.4.1691

Problem:    When switching to a new buffer and an autocommand applies syntax
            highlighting an ml_get error may occur.
Solution:   Check "syn_buf" against the buffer in the window. (Alexander von
            Buddenbrock, closes #676)

src/syntax.c

@@ -509,7 +509,9 @@ syntax_start(win_T *wp, linenr_T lnum)
      * Also do this when a change was made, the current state may be invalid
      * then.
      */
-    if (syn_block != wp->w_s || changedtick != syn_buf->b_changedtick)
+    if (syn_block != wp->w_s
+	    || syn_buf != wp->w_buffer
+	    || changedtick != syn_buf->b_changedtick)
     {
 	invalidate_current_state();
 	syn_buf = wp->w_buffer;

src/version.c

@@ -748,6 +748,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1691,
 /**/
     1690,
 /**/