test/compress: fix buffer overflow

Fixes stack buffer overflow bug in compressdev autotest, which
was caused by the use of buf_idx in the debug logs. Originally, buf_idx
was treated as an array instead of the reference of an integer.
This was fixed by replacing the use of buf_idx[priv_data->orig_idx] with
the variable i.

Fixes: 466a2c4bb5f4 ("test/compress: improve debug logs")
Fixes: 6bbc5a923625 ("test/compress: refactor unit tests")
Cc: stable@dpdk.org
Signed-off-by: Rebecca Troy <rebecca.troy@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>