Fixed wrong range test code for HEAP tables. This caused a crash when doing a range test with a key that didn't have lower or upper bound (Bug #6082)
More test cases



mysql-test/r/heap.result:
  Test for bug #6082 (delete with NOT NULL)
mysql-test/r/key.result:
  More tests for BUG#6151 - myisam index corruption
mysql-test/r/ps.result:
  Test of bug #6047 "Permission problem when executing mysql_stmt_execute with derived table"
mysql-test/t/heap.test:
  Test for bug #6082 (delete with NOT NULL)
mysql-test/t/key.test:
  More tests for BUG#6151 - myisam index corruption
mysql-test/t/ps.test:
  Test of bug #6047 "Permission problem when executing mysql_stmt_execute with derived table"
sql/ha_heap.cc:
  Fixed wrong range test code for HEAP tables. This caused a crash when doing a range test with a key that didn't have lower or upper bound