This is the race between DELETE and INSERT (or other any two operations accessing to the table).
What should happen in good case:
1. ALTER TABLE is issued. vc_templ->default_rec is initialized with temporary share's default_fields
2. temporary share is freed, but datadict is still there, with garbage in vc_templ->default_rec
3. DELETE is issued. It is first after ALTER TABLE finished.
4. ha_innobase::open() is called, ib_table->get_ref_count() should be one
5. we reinitialize vc_templ, so no garbage anymore

What actually happens:
3. DELETE is issued.
4. ha_innobase::open() is called and ib_table->get_ref_count() is 1
5. INSERT (or SELECT etc.) is issued in parallel
6. ha_innobase::open() is called and ib_table->get_ref_count() is 1
7. we check ib_table->get_ref_count()  and it is 2 in both threads when we want reinitialize vc_templ
8. garbage is there

Fix:
* Do not store pointers to SHARE memory in table dict, copy it instead.
* But then we don't need to refresh it each time when refcount=1.