add `isValidPORTEnvVar` function to judge the PORT whether is valid

[bestgopher]

If an addr parameter is not passed in Run, it will get PORT env variable. If there is not a PORT variable，it will use default port - 8080. But sometimes, there is a PORT env variable, but it is not a valid value(0-65535),the app will panic. So I add a function to judge the PORT whether is valid. For example:

my code:

func main() {
    r := gin.Default()
    ...
    if  err := r.Run(); err != nil {
       ...
   }
}

run:

➜  gin $ echo $PORT
abc
➜  gin $ go run main.go
[GIN-debug] [WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.

[GIN-debug] [WARNING] Running in "debug" mode. Switch to "release" mode in production.
 - using env:	export GIN_MODE=release
 - using code:	gin.SetMode(gin.ReleaseMode)

[GIN-debug] GET    /get                      --> main.main.func1 (3 handlers)
[GIN-debug] Environment variable PORT="abcd"
[GIN-debug] Listening and serving HTTP on :abcd
[GIN-debug] [ERROR] listen tcp: lookup tcp/abcd: nodename nor servname provided, or not known
2020/04/16 23:33:36 listen tcp: lookup tcp/abcd: nodename nor servname provided, or not known
exit status 1

[codecov[bot]]

Codecov Report

Merging #2327 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2327   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files          41       41           
  Lines        2304     2308    +4     
=======================================
+ Hits         2267     2271    +4     
  Misses         21       21           
  Partials       16       16           

Impacted Files	Coverage Δ
utils.go	97.53% <100.00%> (+0.12%)	:arrow_up:

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