This change allows you to call `Decode()` on an sbe struct where the struct has
pre-allocated slices. Before, the code would allocate a new slice if the
capacity on the existing slice was less than the number of groups in the
message. If the old slice had a greater capacity, it wouldn't truncate the
number of elements to the new length.

This tests that if the existing decoded struct has a vardata value with
length = 5, and the message to decode has length = 4, then the vardata
value is truncated to length 4.