Remove expensive assert in --rsyncable hot loop

Remove expensive assert in --rsyncable hot loop

This assert slows the loop down by 10x. We can get similar
coverage by asserting at the beginning & end of the loop.

We need this fix because Debian compiles zstd with asserts
enabled. Separately, we should ask them why, and if they would
consider disabling asserts in their builds. Since we don't
optimize for assert enabled builds.

Fixes Issue #3150.