fastboot: Handle libsparse failures better.

sparse_file_len is not actually infallible; on Windows it's pretty easy
to make it fail by embedding large files in the stream. fastboot didn't
handle this anywhere, leading to bad sparse images when libsparse
runs out of address space.

Bug: 273933042
Bug: 268872725
Test: fastboot flashall on Windows
Change-Id: Ie68aed2f1970e820350d9f97aa89a6c0242229b8