tcp: accept FIN on zero window.

[Dirbaio]

No description provided.

[codecov]

Codecov Report

Merging #845 (284a2c9) into main (aa5d887) will decrease coverage by 0.03%.
Report is 61 commits behind head on main.
The diff coverage is 59.50%.

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
- Coverage   79.65%   79.63%   -0.03%     
==========================================
  Files          71       78       +7     
  Lines       27786    27878      +92     
==========================================
+ Hits        22134    22201      +67     
- Misses       5652     5677      +25     

Files	Coverage Δ
src/iface/interface/ipv6.rs	91.38% <100.00%> (+0.12%)	⬆️
src/socket/tcp.rs	96.69% <100.00%> (+0.20%)	⬆️
src/iface/interface/ipv4.rs	68.50% <93.75%> (+0.53%)	⬆️
src/iface/interface/sixlowpan.rs	87.61% <93.75%> (+12.68%)	⬆️
src/iface/interface/mod.rs	45.63% <10.81%> (-3.86%)	⬇️
src/iface/ip_packet.rs	47.05% <44.24%> (ø)

... and 23 files with indirect coverage changes

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