transport: don't add connection to multiplexer if init fails

[kelmenhorst]

This diff removes a net.PacketConn from the multiplexer when (*Transport).init fails.

Background: As far as I am aware, a PacketConn is only removed from the multiplexer, when (*Transport).listen returns.
If (*Transport).init returns early, the listen() goroutine is not started, while the PacketConn was already added to the multiplexer, which results in the inability of the Transport to remove the PacketConn entry from the multiplexer.

[codecov]

Codecov Report

Merging #3931 (dce7555) into master (435444a) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3931      +/-   ##
==========================================
+ Coverage   83.04%   83.15%   +0.11%     
==========================================
  Files         145      145              
  Lines       14595    14594       -1     
==========================================
+ Hits        12120    12135      +15     
+ Misses       1990     1979      -11     
+ Partials      485      480       -5     

Impacted Files	Coverage Δ
transport.go	64.21% <100.00%> (+1.34%)	⬆️

... and 4 files with indirect coverage changes

[marten-seemann]

Have you considered calling getMultiplexer().AddConn later, after the code paths that can error (maybe even directly before t.listen)?

[kelmenhorst]

Thanks! Yes, I think your suggestion makes sense, I will change it accordingly.