fix: panicing on options parsing.

[ldez]

Fix panic when option have a len of 8 on attempts parsing.

panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/containous/traefik/vendor/github.com/miekg/dns.ClientConfigFromFile(0x211b1eb, 0x10, 0x0, 0x0, 0x0)
      /go/src/github.com/containous/traefik/vendor/github.com/miekg/dns/clientconfig.go:86 +0xad6
github.com/containous/traefik/vendor/github.com/xenolf/lego/acme.getNameservers(0x211b1eb, 0x10, 0x32e9920, 0x2, 0x2, 0x92d9bc, 0xc4202240e0, 0x1)
      /go/src/github.com/containous/traefik/vendor/github.com/xenolf/lego/acme/dns_challenge.go:40 +0x4d
github.com/containous/traefik/vendor/github.com/xenolf/lego/acme.init()
      /go/src/github.com/containous/traefik/vendor/github.com/xenolf/lego/acme/dns_challenge.go:33 +0x12d
github.com/containous/traefik/acme.init()
      <autogenerated>:1 +0x84
main.init()
      <autogenerated>:1 +0x7f

[codecov-io]

Codecov Report

Merging #642 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
+ Coverage   57.89%   57.94%   +0.05%     
==========================================
  Files          37       37              
  Lines        9989     9989              
==========================================
+ Hits         5783     5788       +5     
+ Misses       3157     3152       -5     
  Partials     1049     1049

Impacted Files	Coverage Δ
clientconfig.go	78.08% <100%> (+6.84%)	⬆️

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