fix(fs_actions): check window valid on close buf

[levouh]

Same thing as #644 but when removing an item rather than closing it.

Checked a few other places that window operations are done (specifically nvim_win_get_buf) and most don't have as simple of a path forward if the window is not valid, or apply to the Neotree window itself where we are often sure beforehand that it is indeed valid.

[codecov]

Codecov Report

Merging #658 (6a7a19f) into main (c5a5a3d) will increase coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #658      +/-   ##
==========================================
+ Coverage   50.21%   50.29%   +0.08%     
==========================================
  Files          47       47              
  Lines        6227     6227              
==========================================
+ Hits         3127     3132       +5     
+ Misses       3100     3095       -5     

Impacted Files	Coverage Δ
lua/neo-tree/sources/filesystem/lib/fs_actions.lua	3.84% <0.00%> (ø)
lua/neo-tree/utils.lua	46.05% <0.00%> (-0.26%)	⬇️
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%)	⬆️
lua/neo-tree/sources/filesystem/init.lua	66.10% <0.00%> (+1.69%)	⬆️

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