Problem:
If a server is slow with catching up, there can be stale diagnostics
for deleted lines. Then if a user uses `jump` it can error like:
E5108: Lua: ...runtime/lua/vim/diagnostic.lua:670: attempt to index a nil value
stack traceback:
...runtime/lua/vim/diagnostic.lua:670: in function 'get_logical_pos'
...runtime/lua/vim/diagnostic.lua:687: in function 'diagnostic_lines'
...runtime/lua/vim/diagnostic.lua:1122: in function 'next_diagnostic'
...runtime/lua/vim/diagnostic.lua:1665: in function 'jump'
Solution:
Fallback to diagnostic location. That's better than the failure.
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