Fix MSAN in vectorized casting evaluator

Reference issue

Fixes #2928 (closed)

What does this implement/fix?

Some casting functions convert several input packets to a single output packet. For packet segment, it is possible for an entire packet to be unused, which is eventually truncated. However, intermediate operations on that uninitialized data can trigger MSAN errors, and probably invoke UB. This MR explicitly zeros out all those packets.

Additional information