Ensure file exist before checking segment

[manawasp]

Hello, attempt to fix #59.

Context: First thing, the link which generate the error is /products/desktop/11/release-notes/#low-quality ~ l.3032 from @miltador.

Process: htmltest will enter to the method checkInternalHash then to refDoc, _ := hT.documentStore.ResolveRef(ref) which return nil, false because there is no match (method func (dS *DocumentStore) ResolvePath(refPath string) (*Document, bool) ), but methods are called from the struct and one of them try to acces to one of these properties as the mutex, and panic.

Fix: To prevent this error I suggest to check segment only if the file exist

note: naming segment would be not better than hash in checkInternalHash ?

[codecov-io]

Codecov Report

Merging #65 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   95.67%   95.69%   +0.01%     
==========================================
  Files          17       17              
  Lines         902      905       +3     
==========================================
+ Hits          863      866       +3     
  Misses         34       34              
  Partials        5        5

Impacted Files	Coverage Δ
htmltest/check-link.go	95.16% <100%> (+0.05%)	⬆️

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[wjdp]

Fantastic, thanks for this!