fastboot: sparse_file_len() returns int64_t.

Check that the value fits in uint32_t that's supported by the current
protocol.

Also fix and sanity check the max_size before passing it to
sparse_file_resparse(), which accepts `unsigned int`. This shouldn't
happen in practice because of RESPARSE_LIMIT (1 GiB).

Test: `fastboot flash` with small and large images.
Change-Id: I0a8279fc14c54c40a70ddce65c3b25173c0d0a40