patch 9.1.0643: terminal: cursor may end up on invalid position

Problem:  terminal: cursor may end up on invalid position after reducing
          the scrollback lines (user202729)
Solution: After reducing the scrollback size, check the cursor position,
          making sure it does not end up on an invalid line

fixes: #15351

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/terminal.c

@@ -3451,6 +3451,10 @@ limit_scrollback(term_T *term, garray_T *gap, int update_buffer)
 	    sizeof(sb_line_T) * gap->ga_len);
     if (update_buffer)
 	term->tl_scrollback_scrolled -= todo;
+
+    // make sure cursor is on a valid line
+    if (curbuf == term->tl_buffer)
+	check_cursor();
 }
 
 /*

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    643,
 /**/
     642,
 /**/