7zip: propagate skip_stream's actual error code in read_data_skip

7zip: propagate skip_stream's actual error code in read_data_skip

archive_read_format_7zip_read_data_skip used to coerce any negative
skip_stream() return into ARCHIVE_FATAL.  That is wrong in principle:
ARCHIVE_FAILED can legitimately propagate up from setup_decode_folder()
through read_stream() and skip_stream(), and the wrapper should not
upgrade it.

In the current encryption-partially test case this is empirically a
no-op because skip_stream() still returns ARCHIVE_FATAL via a second,
deeper code path through extract_pack_stream().  An inline TODO comment
flags that asymmetry for a follow-up audit.