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 0fd322f812,
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.
e6edcfa795