patch 8.2.1702: crash when using undo after deleting folded lines

Problem:    Crash when using undo after deleting folded lines.
Solution:   Check for NULL pointer. (closes #6968)

src/fold.c

@@ -2422,8 +2422,8 @@ foldUpdateIEMSRecurse(
 							      && flp->lvl > 0)
     {
 	(void)foldFind(gap, startlnum - 1, &fp);
-	if (fp >= ((fold_T *)gap->ga_data) + gap->ga_len
+	if (fp != NULL && (fp >= ((fold_T *)gap->ga_data) + gap->ga_len
-						   || fp->fd_top >= startlnum)
+						   || fp->fd_top >= startlnum))
 	    fp = NULL;
     }
 

src/testdir/test_fold.vim

@@ -816,4 +816,23 @@ func Test_fold_expr_error()
   close!
 endfunc
 
+func Test_undo_fold_deletion()
+  new
+  set fdm=marker
+  let lines =<< trim END
+      " {{{
+      " }}}1
+      " {{{
+  END
+  call setline(1, lines)
+  3d
+  g/"/d
+  undo
+  redo
+  eval getline(1, '$')->assert_equal([''])
+
+  set fdm&vim
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1702,
 /**/
     1701,
 /**/