fix: check err returned by newGenericDecompressor

There is 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 commit adds check for `err` to prevent dereferencing potentially
nullable decompressor.

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

Signed-off-by: Mikhail Dmitrichenko <m.dmitrichenko222@gmail.com>