backport: fix(lua): verify buffer in highlight.on_yank (#15482)

Resolve an issue with deferred clearing of highlight failing if the
buffer is deleted before the timeout by checking whether the
buffer is valid first.

test/functional/lua/highlight_spec.lua

@@ -0,0 +1,26 @@
+local helpers = require('test.functional.helpers')(after_each)
+local funcs = helpers.funcs
+local exec_lua = helpers.exec_lua
+local command = helpers.command
+local clear = helpers.clear
+
+describe('vim.highlight.on_yank', function()
+
+  before_each(function()
+    clear()
+  end)
+
+  it('does not show errors even if buffer is wiped before timeout', function()
+    command('new')
+    local bufnr = funcs.bufnr("%")
+    exec_lua[[
+      vim.highlight.on_yank({timeout = 10, on_macro = true, event = {operator = "y", regtype = "v"}})
+      vim.cmd('bwipeout!')
+    ]]
+    exec_lua[[vim.wait(10)]]
+    local pattern = [[vim/highlight.lua:%d+: Invalid buffer id: ]] .. bufnr
+    local exists = pcall(helpers.assert_log, pattern)
+    assert.is_false(exists, string.format("%q should not be in log", pattern))
+  end)
+
+end)