Fix 'ranger-revert' not killing buffers behavior:

[ANDRON94]

• during the "Reverting all buffers" stage of 'ranger-revert' function 'ranger-frame-exists-p' predicate returned '(nil)' (which is evaluated to "true" in Lisp) but 'nil' was expected.

The behavior was noticed during testing quit ('q' keybinding in ranger's mode map) function. Steps to reproduce:

1. Open any directory with ranger.
2. Press 'q'.

Expected:

1. All ranger related buffers are killed (with 'ranger-cleanup-on-disable' set to non-nil).

Actual:

1. All ranger related buffers stay alive.