fix(launch): check buffer is valid before get_name

[coffebar]

I'm using the plugin, that manages sessions using builtin :mksession and race lead to error on line 118, where
api.nvim_buf_get_name(bufnr) called on not existing buffer.
This PR adds check api.nvim_buf_is_valid to fix this error.

This fix works on this line, but sometimes I see also an error message from neovim itself:
[LSP] No client with id:

it should be fixed too. But this is not related to nvim-lspconfig