This patch updates the 'bl31_check_ns_address()' helper function to
check that the memory address and size passed by the NS world are not
zero.

The helper fucntion also returns the error code as soon as it detects
inconsistencies, to avoid multiple error paths from kicking in for the
same input parameters.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

Change-Id: I46264f913954614bedcbde12e47ea0c70cd19be0