libfdisk: Include table-length in first-lba checks

libfdisk: Include table-length in first-lba checks

Otherwise, sfdisk would error out on its own dump, as first-lba would be
earlier than "plausible" assuming the default table size.

This is because it did not look for `table-length` when using an sfdisk
script. This means that using `--dump` on a valid image, could not be
unserialized back into a valid image.

This is helpful in exotic situations, where a partition has to be
created earlier than the conventional "first-lba" (2048). For example,
on Allwinner A64 boards, where a firmware is stored at LBA16. With this
it is possible to ask for `first-lba` at or earlier than 16, by reducing
the number of entries to e.g. 48.