fix(fs): check window is valid before replacing buffers

[levouh]

Fix #632

Admittedly I did not do much debugging to find out what the actual issue is (e.g. are we assuming that the window should be valid, and it not being messes with something else?), however in any case it seems logical to check first that the window we are targeting is indeed valid before trying to call functions on it.

[codecov]

Codecov Report

Merging #644 (3d8ac46) into main (47b8d1b) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   50.57%   50.60%   +0.03%     
==========================================
  Files          47       47              
  Lines        6187     6187              
==========================================
+ Hits         3129     3131       +2     
+ Misses       3058     3056       -2     

Impacted Files	Coverage Δ
lua/neo-tree/sources/filesystem/lib/fs_actions.lua	3.98% <0.00%> (ø)
lua/neo-tree/sources/manager.lua	61.09% <0.00%> (+0.30%)	⬆️
lua/neo-tree/sources/buffers/init.lua	71.00% <0.00%> (+1.00%)	⬆️

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