Address warnings about possible division by zero from clangsa

Address warnings about possible division by zero from clangsa

* The complaint in ztest_replay_write() is only possible if something
   went horribly wrong. An assertion will silence this and if it goes
   off, we will know that something is wrong.
 * The complaint in spa_estimate_metaslabs_to_flush() is not impossible,
   but seems very unlikely. We resolve this by passing the value from
   the `MIN()` that does not go to infinity when the variable is zero.

There was a third report from Clang's scan-build, but that was a
definite false positive and disappeared when checked again through
Clang's static analyzer with Z3 refution via CodeChecker.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14124