boot: bootutil: require exact hash TLV length

[d3zd3z]

boot_save_boot_status() reads the image hash out of the manifest into a
fixed-size stack buffer, uint8_t image_hash[IMAGE_HASH_SIZE]. The only length
check on the hash TLV was an upper bound:

if (len > sizeof(image_hash)) {
    return -1;
}
rc = flash_area_read(fap, offset, image_hash, len);

So a TLV shorter than the configured hash length was accepted, and
flash_area_read() filled only the first len bytes. The rest of image_hash
was left holding whatever happened to be on the stack.

Further down, the copy into the boot record uses the buffer size rather than
len:

memcpy(buf + offset, image_hash, sizeof(image_hash));

so that stale stack content is copied into the record that
boot_add_data_to_shared_area() places in the shared data area, and a
consumer of the record sees it as part of the measurement.

A hash TLV shorter than the hash size is malformed, so this rejects it outright
instead of reading it partially. bootutil_img_validate() already applies the
same exact-length check (if (len != sizeof(hash))), so an image that passes
validation is unaffected by this change; it only tightens the path taken when
the slot is not hash-validated.

Only compiled under MCUBOOT_MEASURED_BOOT.

Testing: cargo test in sim/ passes (25 passed, 0 failed). Note that the sim
does not build boot/bootutil/src/boot_record.c and never defines
MCUBOOT_MEASURED_BOOT, so that is a no-regression run rather than coverage of
this code; the edited file was additionally compiled standalone with measured
boot enabled, with no diagnostics.

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