[RFC] Add FOR_ALL_BUFFERS helper

There is `FOR_ALL_WINDOWS` and `FOR_ALL_TAB_WINDOWS` thought `FOR_ALL_BUFFERS` might be handy as well.
This `buf` is referenced later and if the next `for` doesn't iterate at least once, it will be mistakenly dereferenced/compared. Initialize it as `NULL`. Event if the `for` is guaranteed to run at least once, the static analysis tools we use may see it as a potential bug.