fix: potential endless loop under extremely high load

[sudden6]

If there are more connections waiting to be accepted than MAX_INCOMING_CONNECTIONS existing accepted connections are overwritten.

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This change is 

[codecov]

Codecov Report

Merging #2364 (8738960) into master (9fae455) will decrease coverage by 68.15%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##           master   #2364       +/-   ##
==========================================
- Coverage   78.10%   9.95%   -68.16%     
==========================================
  Files         141     141               
  Lines       31114   32503     +1389     
==========================================
- Hits        24302    3236    -21066     
- Misses       6812   29267    +22455     

Impacted Files	Coverage Δ
toxcore/TCP_common.c	6.38% <0.00%> (-72.14%)	⬇️
toxcore/TCP_server.c	8.12% <0.00%> (-70.65%)	⬇️
toxcore/tox.c	7.80% <0.00%> (-56.32%)	⬇️
toxcore/crypto_core.c	33.88% <33.33%> (-59.83%)	⬇️
toxav/toxav_old.c	0.00% <0.00%> (-100.00%)	⬇️
auto_tests/file_saving_test.c	0.00% <0.00%> (-100.00%)	⬇️
auto_tests/conference_two_test.c	0.00% <0.00%> (-100.00%)	⬇️
auto_tests/invalid_tcp_proxy_test.c	0.00% <0.00%> (-100.00%)	⬇️
auto_tests/invalid_udp_proxy_test.c	0.00% <0.00%> (-100.00%)	⬇️
auto_tests/group_message_test.c	5.59% <0.00%> (-93.66%)	⬇️
... and 133 more

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