Validate command headers before dispatch

[mjc]

Reject truncated app-command payloads before reading their two-byte header.

The VESC app-data callback can receive zero- or one-byte payloads. The old handler read buffer[0] and buffer[1] before checking the length, producing an out-of-bounds read. The old length check then returned for len < 2, so these packets did not reach command dispatch.

The guard now runs before either header byte is accessed.

I was able to eventually get a device reboot from this but it took a lot of packets.

[Copilot]

Pull request overview

This PR hardens the VESC app-command receive path by validating the minimum payload length before reading the two-byte command header, preventing out-of-bounds reads on truncated (0–1 byte) packets.

Changes:

• Move the len < 2 guard to run before accessing buffer[0]/buffer[1].
• Keep existing command dispatch behavior unchanged for valid packets while rejecting too-short payloads early.

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