Fix: validate register write byte counts

[MrAlaskan]

Problem

Modbus function codes 0x10 (Write Multiple Registers) and 0x17 (Read/Write Multiple Registers) require the write byte count to equal twice the write quantity, because every register occupies two bytes.

The request decoders did not enforce this relationship. For 0x10, decoding was based on the quantity while the declared byte count was discarded, which could cause extra data to be ignored or truncated data to raise a parsing error. For 0x17, registers were decoded using the byte count, but a byte count inconsistent with the write quantity could still reach the datastore. As a result, malformed requests could be accepted, incorrectly parsed, or fail without returning the required Illegal Data Value (03) response.

Changes

• Preserve the declared byte count and actual payload length for decoded 0x10 and 0x17 requests, validate both before accessing the datastore, and return Illegal Data Value (03) when they are inconsistent. Parsing now consumes only complete register values so malformed payloads can be rejected cleanly instead of raising struct.error.
• Add regression tests covering byte counts that are too small, too large, odd, shorter than the actual payload, or followed by extra data. The tests also verify that malformed requests do not write to the datastore and that valid requests continue to succeed.

[janiversen]

Please solve formatting problem (ubuntu 3.14).

Please use ./check_ci.sh, that checks and reformats just like CI, and afterward commit changed files.

[janiversen]

Why make an extra split ? Line 154 could just use data directly.

[MrAlaskan]

Done. The registers are now unpacked directly from data. I applied the same simplification to both 0x17 and 0x10.

[janiversen]

LGTM, thanks.