sd-device: fix double-free
    
    If an attribute is read but the value is not used (i.e. ret_value is NULL),
    then sd_device_get_sysattr_value() mistakenly frees the read data even though
    it is cached internally.
    
    Fixes a bug introduced by acfc2a1d15560084e077ffb3be472cd117e9020a.
    
    Fixes #25702.
    
    (cherry picked from commit eb18e7b7825e8320bb4d6269690ef8c3f5461d2b)