fix(core): ensure proper handling of vendor string

[cepetr]

This PR fixes a bug introduced in #4188 that caused an invalid read of the vendor string from the vendor header.

Before the fix, trezorctl get-features printed:

...

fw_vendor: 'UNSAFE, DO NOT USE!TOIfx',

...

No changelog entry is needed since this bug wasn't released yet.

[bosomt]

QA OK

trezorctl get-features
Features (212 bytes) {
    bootloader_locked: True,
    busy: False,
    device_id: '36214063F33A7BA5825E8DEC',
    fw_vendor: 'DEV ONLY, DO NOT USE!',
    haptic_feedback: True,
    homescreen_format: Jpeg (2),
    homescreen_height: 240,
    homescreen_width: 240,
    initialized: True,
    internal_model: 'T3T1',
    label: '284 acad',
    language: 'en-US',
    language_version_matches: True,
    major_version: 2,
    minor_version: 8,
    model: 'Safe 5',
    optiga_sec: 0,
    patch_version: 4,
    pin_protection: True,
    revision: 20 bytes 0xd71d9e9c343ad04a7f28d6b02f8d3f8945af30d6,