[mojo-edk] Only zero the header in a new Channel::Message.
[mojo-edk] Only zero the header in a new Channel::Message.

The payload is expected to be overwritten, so the memset is redundant.
If sanitizers start to complain, this should be considered an issue with
how the payload is written and read, and fixed appropriately. It is
almost certainly not appropriate to fix those errors by extending the message
creation to zero out the payload.

BUG=None

Review URL: https://codereview.chromium.org/1768053002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379661}

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