When there is an error executing EXISTS predicates they return NULL as their string
or decimal value but don't set the NULL value flag.
Fixed by returning 0 (as a decimal or a string) on error exectuting the subquery.
Note that we can't return NULL as EXISTS is not supposed to return NULL.

mysql-test/r/subselect.result:
  Bug#37627: test case
mysql-test/t/subselect.test:
  Bug#37627: test case
sql/item_subselect.cc:
  Bug#37627: return decimal (or string) 0 isntead of a NULL pointer on
  error calculating an EXISTS predicate.