Fix null pointer dereference in ParseCommitWithSignature

[SagePtr]

Fixes #4961

Explanation: tag commits do not have 'author' and 'committer', they have 'tagger' field (which is treated as 'author' in vendor/code.gitea.io/git/repo_commit.go:68) and committer is simply nil.

But ParseCommitWithSignature thinked if signature is set - then committer is set too. So now it just skips checking signature if no committer is set.

Probably repo_commit.go from vendor git module should set 'tagger' to both 'author' and 'committer' fields, I have no idea what behaviour is correct (addressed here: go-gitea/git#129 )

Or maybe ParseCommitWithSignature should check signature against both Author and Committer and return true if any one of them is correct.

Now I just check against null pointer to solve panic issue.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8b113cd). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #4962   +/-   ##
========================================
  Coverage          ?   37.4%           
========================================
  Files             ?     306           
  Lines             ?   45334           
  Branches          ?       0           
========================================
  Hits              ?   16959           
  Misses            ?   25922           
  Partials          ?    2453

Impacted Files	Coverage Δ
models/gpg_key.go	55.23% <100%> (ø)

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[lunny]

@SagePtr please send a back port to release/v1.5