It's perfectly fine to have blocked interrupts when
ProcessClientWriteInterrupt() is called. In fact it's commonly the
case when emitting error reports. And we deal with that correctly.

Even if that'd not be the case, it'd be a bad location for such a
assertion. Because ProcessClientWriteInterrupt() is only called when
the socket is blocked it's hard to hit.

Per Heikki and buildfarm animals nightjar and dunlin.