patch 8.1.2168: heredoc assignment not skipped in if block

Problem:    Heredoc assignment not skipped in if block.
Solution:   Check if "skip" is set. (closes #5063)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 3bb5748..64d178c 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -295,4 +295,12 @@
   app
   END
   call assert_equal(['something', 'app'], var1)
+
+  let check = []
+  if 0
+     let check =<< trim END
+       from heredoc
+     END
+  endif
+  call assert_equal([], check)
 endfunc