The convert_constant_item function converts a constant to integer using
field for condition like 'field = a_constant'. In some cases the
convert_constant_item is called for a subquery when outer select is already
being executed, so convert_constant_item saves field's value to prevent its
corruption. For EXPLAIN and at the prepare phase field's value isn't
initialized yet, thus when convert_constant_item tries to restore saved
value it fails assertion.

Now the convert_constant_item doesn't save/restore field's value if it's
haven't been read yet. Outer constant values are always saved.

mysql-test/r/explain.result:
  Added a test case for the bug#37870.
mysql-test/t/explain.test:
  Added a test case for the bug#37870.
sql/item_cmpfunc.cc:
  Bug#37870: Usage of uninitialized value caused failed assertion.
  Now the convert_constant_item doesn't save/restore field's value if it's
  haven't been read yet. Outer constant values are always saved.