main: don't access msgbs after giving them away
Because of IRQ, it is dangerous to access a msgb,
which has been giving away to a queue (ownership transfer).
Further OSMO_ASSERT() should never fail on a queue'd object,
except it has been taken already from an IRQ context.
Fix a potential crash.
Fixes: a684bc4e38b4 ("Make ch9 usb tests work")
Change-Id: I79844f77d5cd75e08b0eb44b22c4ad223cb79dcb
