The class Copy_field contains a String tmp, 
which may allocate memory on the heap.
That means that all instances of Copy_field
must be properly destroyed. Alas they are not.

Solution: don't use Copy_field::tmp for copying
from_field => tmp => to_field
in do_field_string()


sql/field.cc:
  In Field_set::val_str
  return empty string (of appropriate character set) for an empty set.
sql/field.h:
  New private member in Field_enum: empty_set_string.
sql/field_conv.cc:
  In do_field_string, use an auto variable for copying
  from_field => tmp => to_field
  rather than copy->tmp.