Problem:
`vim.diagnostic.handlers.underline.show` throws `E565: Index out of bounds` when
it tries to underline a diagnostic whose `lnum` is past the end of the buffer.
This happens with stale diagnostics set on an unloaded buffer (e.g. via
`bufadd()`) that are drawn only after the buffer is loaded: by then the file on
disk can be shorter than the `lnum` the diagnostic carried. File pickers
(snacks.nvim) that open files via `bufadd` + `:buffer` hit this whenever an LSP
server has already emitted diagnostics for that URI.
Solution:
Skip diagnostics with an out-of-range `lnum` in the underline handler.
Other handlers `M.virtual_text.show()`, `M.signs.show()`, have a similar condition.
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