MdeModulePkg/HiiDatabaseDxe: Add ASSERT before using the pointer 'String'

MdeModulePkg/HiiDatabaseDxe: Add ASSERT before using the pointer 'String'

The 'Sting' is returned by the function GetUnicodeStringTextAndSize.
If it is NULL, function GetUnicodeStringTextAndSize will return
EFI_OUT_OF_RESOURCES, and error handling codes will cover it.
So the pointer 'Sting' can not be NULL when using it.
So we can add the ASSERT codes.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>