check err returned by newGenericDecompressor

[grey3228]

Hello!

I used SAST tool Svace and found a couple of newGenericDecompressor function usages, where returned possibly non-nil err is not checked before dereferencing returned decompressor. It may lead to nil ptr dereferencing.

This PR adds check for err to prevent dereferencing potentially nullable decompressor.

Found by Linux Verification Center (linuxtesting.org) with SVACE

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Does this PR introduce a user-facing change?

None

[baude]

LGTM, honestly, took me a minute to get it before it was a facepalm

[baude]

I've added the no-test label and restarted tests.

[mheon]

The return should probably be changed to return nil instead of err, given err is now guaranteed to be nil... but that's not worth re-pushing over.
/lgtm
/approve

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This pull-request has been approved by: grey3228, mheon

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