Only verify size if we can

[jonjohnsonjr]

Sometimes Content-Length isn't set, and sometimes we don't know what it
should be. Do our best regardless.

Fixes #1061

[imjasonh]

How do you feel about const SizeUnknown = -1

[codecov-commenter]

Codecov Report

Merging #1080 (a3b65cf) into main (5ea3569) will increase coverage by 0.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main    #1080      +/-   ##
==========================================
+ Coverage   75.51%   75.52%   +0.01%     
==========================================
  Files         107      107              
  Lines        5096     5100       +4     
==========================================
+ Hits         3848     3852       +4     
  Misses        703      703              
  Partials      545      545              

Impacted Files	Coverage Δ
pkg/v1/remote/descriptor.go	73.65% <60.00%> (+0.28%)	⬆️
internal/verify/verify.go	100.00% <100.00%> (ø)
pkg/v1/remote/image.go	79.72% <100.00%> (ø)
pkg/v1/remote/layer.go	65.51% <100.00%> (ø)

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[jonjohnsonjr]

How do you feel about const SizeUnknown = -1

I love it, done.

[jonjohnsonjr]

PTAL