[legacy] Fix bug in zstd-0.5 decoder

[legacy] Fix bug in zstd-0.5 decoder

The match length and literal length extra bytes could either
by 2 bytes or 3 bytes in version 0.5. All earlier verions were
always 3 bytes, and later version didn't have dumps.

The bug, introduced by commit 0fd322f812211e653a83492c0c114b933f8b6bc5,
was triggered when the last dump was a 2-byte dump, because we didn't
separate that case from a 3-byte dump, and thought we were over-reading.

I've tested this fix with every zstd version < 1.0.0 on the buggy file,
and we are now always successfully decompressing with the right
checksum.

Fixes #1693.