[Bug] Fix BE coredump for 'ferror(nullptr)' in UserFunctionCache

[weizuo93]

Proposed changes

Fix #6330

In function doris::UserFunctionCache::_download_lib(), ferror(fp.get()) will trigger BE coredump if it failed to open file tmp_file and fpis nullptr.

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[morningman]

LGTM

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PR approved by at least one committer and no changes requested.

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