Make sure to close streams when done with them.

https://github.com/simtel12/zip-signer/commit/488f8cd3b466e2da2d9dc96dafaeef1d67e005f7

closes #1383
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f