Bug 6253 followup: explicitly check ref value for ARRAY before casting

Bug 6253 followup: explicitly check ref value for ARRAY before casting

in C4::SQLHelper::_Process_Operands, $operand has been modified to be cast as an array ref
if the incoming $operand is not a ref.  However, the incoming $operand could be some other
kind of ref (hashref for example), which would case the dereferencing in line 419 to fail and exit.

Adding an explicit test for whether the ref is ARRAY seems to clear up this issue.

To test, run t/db_dependent/Members.t.  Before, it fails after test 3.  After, it completes all tests.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>