remove unneeded assert causing exit in #35

there was an assert in filechecksummer.cpp to check if the distance
would not become greater than the blocksize.
this assert was before a if statement fixing that 'problem' by setting
distance equal to blocksize when distance is to big.
so the assert can be removed.

this fixes #35

Signed-off-by: BlackEagle <ike.devolder@gmail.com>
tests to make sure #35 does not come back

Signed-off-by: BlackEagle <ike.devolder@gmail.com>