libtrap: bugfix in TCPIP interface

[janskto1]

After a socket was closed because of unsuccessful negotiation,
it was not set to -1. This caused other functions to treat the socket as valid.
Such bahevior resulted in situation where this socket was passed to select() function.
Select() function returns EBADF when a bad file descriptor is passed.
Further fucntions subsequently segfaulted.

[codecov-io]

Codecov Report

Merging #99 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   50.79%   50.78%   -0.02%     
==========================================
  Files          50       50              
  Lines       10231    10232       +1     
==========================================
- Hits         5197     5196       -1     
- Misses       5034     5036       +2

Impacted Files	Coverage Δ
libtrap/src/ifc_tcpip.c	61.06% <0%> (-0.08%)	⬇️
libtrap/src/trap.c	46.02% <0%> (-0.06%)	⬇️

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