Respect helm-buffer-max-length if it's nil

[samertm]

Don't overwrite helm-buffer-max-length when it's nil, because nil indicates that we should use the max length of the longest file every time.

This code is now the same as what helm-buffers does: https://github.com/emacs-helm/helm/blob/8ed0ab7762ad76da02d8832aa18d03e1288ea6d5/helm-buffers.el#L300

You can reproduce the issue by setting helm-buffer-max-length to nil, opening helm projectile, and then opening helm buffer and seeing that the files in the buffer may be getting cut off if the longest file in helm projectile is shorter.

This is just a drive-by PR and I've fixed the problem on my end, so I haven't checked the boxes below. If it's too much extra work on your end, no need to merge. Thanks!!

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Thanks!

[bbatsov]

The changes look good, but your branch has to be rebased on top of the current master branch due to merge conflicts.