Changes

main: fix crash when ccid_df_write_in() fails

The OSMO_ASSERT(msg->list.next != LLIST_POISON1) are checking msgb
which has been removed from the llist.
All llist members which aren't part of a list have LLIST_POISON1 assigned.

These OSMO_ASSERT()s will always trigger this msgb.

Fixes: a684bc4e38b4 ("Make ch9 usb tests work")
Change-Id: I49c81fba756f3620638b6c6c664f53e4a8758ef9

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