fix(intel): assert if bl_mem_params is NULL pointer

This patch fixes the code issue detected by Klocwork scan. Pointer
'bl_mem_params' returned from call to function 'get_bl_mem_params_node'
may be NULL and the NULL pointer may be caused the system crash. Update
the code to assert if unexpected NULL pointer is returned.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Change-Id: I00f3132a6104618cadce26aa303c0b46b5921d5b