fix nil pointer exception when calling Start again after address binding error

[tehsphinx]

There is a nil pointer exception because the code assigns a nil value to an interface (e.Listener) which ends up creating a non-nil interface containing a nil pointer. Using this will result in the nil pointer exception and checking the e.Listener against nil will be false -- which is why the Listener is not recreated and overwritten the during the second call to Start.

Small code example that triggers this error (given an existing echo router):

if err := e.Start(":6060"); err != nil {
	// retry with random port
	if r := e.Start(":"); r != nil {
		log.Fatal(r)
	}
}

[codecov]

Codecov Report

Merging #2131 (8bcff16) into master (b445958) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2131   +/-   ##
=======================================
  Coverage   92.10%   92.11%           
=======================================
  Files          37       37           
  Lines        3028     3030    +2     
=======================================
+ Hits         2789     2791    +2     
  Misses        150      150           
  Partials       89       89           

Impacted Files	Coverage Δ
echo.go	95.17% <100.00%> (+0.03%)	⬆️

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[aldas]

LGTM