Don't assert mysql->mysql is non-null

Don't assert mysql->mysql is non-null

There is an edge case in constructor behavior where we can end up
with mysql->mysql being NULL (rather than mysql itself already being
NULL). I think that ultimately that's a bug in the constructor code,
and we should probably be destroying the outer structure on
construction failure as well. However it's pretty hard to unravel
with when considering all the construction permutations.