onlp/sysi: Make the ONIE IDPROM buffer pass strict_string_checks

[richardkuo1999]

onlp_sysi_onie_data_get() allocated exactly 256 bytes and filled every one of them with raw IDPROM content, so the buffer handed to is_valid_tlvinfo_header__() is not guaranteed to be NUL-terminated. strcmp() requires a terminated string, so AddressSanitizer with strict_string_checks=1 reports a heap-buffer-overflow on that call whenever the device carries no zero byte at all - an erased or corrupted IDPROM. Allocating one extra zero byte makes the same run come back clean.

Real hardware is not affected: on a programmed IDPROM totallen sits at offsets 9-10, so data[9] is 0x00 for any payload below 256 bytes, and the comparison resolves at index 7 or earlier. The change is about satisfying the strcmp contract under strict checking, not about a failure observed in the field.

The read length is deliberately left at 256, so the decoded content and the *size == len guard behave exactly as before.

• introduce a local const int len instead of repeating the literal 256
• derive the i2c word-read loop bound from len (len / 2) on as7716-24xc, as7716-32x and as7816-64x, and the byte-read bound on as9516-32d
• covers 38 platforms; no behavioural change on any of them