tcpheader: Fix BOOST_ASSERT failure in calculate_checksum

tcpheader: Fix BOOST_ASSERT failure in calculate_checksum

The assertion (sizeof(current_size) / 2) >= sizeof(size) was failing on
64-bit systems because:
- current_size was int64_t (8 bytes), so sizeof(current_size)/2 = 4
- size was size_t (8 bytes), so sizeof(size) = 8
- 4 >= 8 is false

Fix by:
- Using size_t for current_size to match the size parameter type
- Simplifying the assertion to check that current_size can hold size
- Simplifying the size decrement since both are now size_t

Test: test_tcpheader/calculate_tcp_checksum_ipv4 now passes