During testing of qemu-riscv32 with a 2GiB memory configuration,
tftp always fails with a error message:

  Load address: 0x84000000
  Loading: #
  TFTP error: trying to overwrite reserved memory...

It turns out the result of 'tftp_load_addr + tftp_load_size' just
overflows (0x100000000) and the test logic in store_block() fails.
Fix this by adjusting the end address to ULONG_MAX when overflow
is detected.

Fixes: a156c47e ("tftp: prevent overwriting reserved memory")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>