pkg/tarsum: fix panic with dynamic buffer

When read is called on a tarsum with a two different read sizes, specifically the second call larger than the first, the dynamic buffer does not get reallocated causing a slice read error.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Merge pull request #8237 from dmcgowan/tarsum_buffer_fix

tarsum: fix panic with dynamic buffer