patch 8.0.0968: crash when switching terminal modes

Problem:    Crash when switching terminal modes. (Nikolai Pavlov)
Solution:   Check that there are scrollback lines.

Author: brammool

src/terminal.c

@@ -1037,14 +1037,13 @@ term_enter_job_mode()
 
     /* Remove the terminal contents from the scrollback and the buffer. */
     gap = &term->tl_scrollback;
-    while (curbuf->b_ml.ml_line_count > term->tl_scrollback_scrolled)
+    while (curbuf->b_ml.ml_line_count > term->tl_scrollback_scrolled
+							    && gap->ga_len > 0)
     {
 	ml_delete(curbuf->b_ml.ml_line_count, FALSE);
 	line = (sb_line_T *)gap->ga_data + gap->ga_len - 1;
 	vim_free(line->sb_cells);
 	--gap->ga_len;
-	if (gap->ga_len == 0)
-	    break;
     }
     check_cursor();
 

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    968,
 /**/
     967,
 /**/