The bug is not related to MERGE table or TRIGGER. More correct description
would be 'assertion on multi-table UPDATE + NATURAL JOIN + MERGEABLE VIEW'.
On PREPARE stage(see test case) we call mark_common_columns() func which
creates ON condition for NATURAL JOIN and sets appropriate
table read_set bitmaps for fields which are used in ON condition.
On EXECUTE stage mark_common_columns() is not called, we set
necessary read_set bitmaps in setup_conds(). But 'B.f1' field
is already processed and related item alredy fixed before
setup_conds() as updated field and setup_conds can not set
read_set bitmap because of that.
The fix is to set read_set bitmap for appropriate table field even
if Item_direct_view_ref item which represents a refernce to this field
is fixed.