Problem: Adding a virtual generated column could cause a deadlock in
debug builds. This was caused by a virtual column purge thread
callback becoming blocked in open_table_uncached() because it could
not acquire the LOCK_open mutex. The mutex, in turn, was needed to
check if the table being opened was present in the TDC.

Solution: Relax condition for assert and associated lock so that it is
possible to call open_table_uncached() without taking LOCK_open or
checking the TDC provided the table will not be opened in SE. Relaxing
the assert is safe in this case as the second share outside the TDC
will not have its own SE private data.