The convert_constant_item function converts a constant to integer using
field for condition like 'field = a_constant'. When the convert_constant_item
is called for a subquery the outer select is already being executed, so
convert_constant_item saves field's value to prevent its corruption.
For EXPLAIN field's value isn't initialized 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
for EXPLAIN.


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
  for EXPLAIN.