The assertion implies the possible transition _keep_alive = 0 -> _keep_alive = 1, which has implication to GC.

Based on comments in JDK-8162553, it was done this way due to null class loader data having a _keep_alive value of 0.

However, we now set null class loader data's _keep_alive = 1 in ClassLoaderData's constructor, so above case should not happen. Let's tighten the assertion to avoid confusion.