[lldb] Assert immediately prior to calling PopPlan

This is part of ongoing attempts to catch the test from
2684281d failing on Arm and AArch64.

I did get logs for the failure but only on Arm, where the backtrace is
truncated. So, let's do the assert that PopPlan was going to do,
before we call it.

Then I should know exactly which PopPlan is asserting.

Technically I should take a mutex here, but technically I shouldn't
be debugging via buildbot, so I'm going to take the risk temporarily.