Fix index-out-of-bounds read when validating PDO mapping

[nattgris]

The number_of_mappings argument to the validation function comes straight from the 8-bit payload when writing to subindex 0 of the PDO mapping objects and will, during the summing of the bitlengths, read outside the mappings array if the payload is greater than MAX_PDO_ENTRIES.

[hefloryd]

Merged, thanks