[WIP] Fix some clang analysis warnings

For the time being I fixed only [two](http://neovim.org/doc/build-reports/clang/report-21342f.html#EndPath) [errors](http://neovim.org/doc/build-reports/clang/report-f0c777.html#EndPath) `Argument with 'nonnull' attribute passed null` in `open_line()` from `misc1.c`.
The problem is that `clang` don't know that `int get_leader_len()` can't return something less than 0.
Better solution could be the change of the return type to `size_t` or `uint32_t` but it would involve more changes and I can't decide which type would be the best. Though if it is desirable I would be glad to realise that change.

I plan to fix some more warnings in the future in this PR.
No idea why my comments aren't appearing the global conversation, but to rephrase them:

- `t > 0` is always true, the type is unsigned (the compiler should have warned about this)
- `count - 1` with an unsigned type is inviting an underflow, even if it can be guaranteed that `count > 1`, we should still rewrite the loop to do without subtractions if possible (or with a signed type).