zmq4: do not panic b/c of invalid connection attempts

[sbinet]

Updates #56.

[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 3.84%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   57.68%   61.53%   +3.84%     
==========================================
  Files          26       26              
  Lines        1933     1934       +1     
==========================================
+ Hits         1115     1190      +75     
+ Misses        726      660      -66     
+ Partials       92       84       -8

Impacted Files	Coverage Δ
socket.go	82.06% <100%> (+3.37%)	⬆️
conn.go	58.94% <100%> (+4.21%)	⬆️
pub.go	72.43% <0%> (+2.16%)	⬆️
router.go	77.77% <0%> (+3.17%)	⬆️
pair.go	73.91% <0%> (+8.69%)	⬆️
sub.go	78.46% <0%> (+9.23%)	⬆️
socket_types.go	92.45% <0%> (+9.43%)	⬆️
req.go	75% <0%> (+14.28%)	⬆️
... and 6 more

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