drivers: shs: Fix potential null dereference on page alloc failure

Check before using page allocated for capabilities, gold flows
and slow start flows.

CRs-fixed: 2576578
Change-Id: I8f062004466447703c84912506af5963035c163c
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>