object-file: use size_t for object size in check_object_signature()

[i80487]

check_object_signature() function prototype defines size parameter as an unsigned long. However, it receives size as size_t from its current callers and forwards it unchanged to hash_object_file(), which expects size_t. Using size_t here keeps the types consistent and avoids an implicit narrowing conversion to unsigned long, which may truncate values when size >= 2^32 on Windows (an unsigned long is 32-bit while a size_t is 64-bit).