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