patch 9.0.0667: ml_get error when 'splitkeep' is "screen"

Problem:    ml_get error when 'splitkeep' is "screen". (Marius Gedminas)
Solution:   Check the botline is not too large. (Luuk van Baal,
            closes #11293, closes #11292)
diff --git a/src/window.c b/src/window.c
index 79ff5a4..1f3f9ad 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6356,7 +6356,8 @@
 	if (wp->w_height != wp->w_prev_height)
 	{
 	    // If window has moved update botline to keep the same screenlines.
-	    if (*p_spk == 's' && wp->w_winrow != wp->w_prev_winrow)
+	    if (*p_spk == 's' && wp->w_winrow != wp->w_prev_winrow
+		      && wp->w_botline - 1 <= wp->w_buffer->b_ml.ml_line_count)
 	    {
 		lnum = wp->w_cursor.lnum;
 		diff = (wp->w_winrow - wp->w_prev_winrow)