patch 9.0.0077: wrong restored cursor position when switching window in autocmd

Problem:    When switching window in autocmd the restored cursor position may
            be wrong.
Solution:   Do not restore the cursor if it was not set. (closes #10775)
diff --git a/src/window.c b/src/window.c
index e7b44ab..83cc24e 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6832,10 +6832,13 @@
     FOR_ALL_TAB_WINDOWS(tp, wp)
 	if (wp->w_buffer == curbuf)
 	{
-	    // Restore the value if the autocommand didn't change it.
-	    if (EQUAL_POS(wp->w_save_cursor.w_cursor_corr, wp->w_cursor))
+	    // Restore the value if the autocommand didn't change it and it was
+	    // set.
+	    if (EQUAL_POS(wp->w_save_cursor.w_cursor_corr, wp->w_cursor)
+				  && wp->w_save_cursor.w_cursor_save.lnum != 0)
 		wp->w_cursor = wp->w_save_cursor.w_cursor_save;
-	    if (wp->w_save_cursor.w_topline_corr == wp->w_topline)
+	    if (wp->w_save_cursor.w_topline_corr == wp->w_topline
+				      && wp->w_save_cursor.w_topline_save != 0)
 		wp->w_topline = wp->w_save_cursor.w_topline_save;
 	}
 }