arm64: early_alloc: Fix check for allocation failure

In early_alloc we check if the memblock_alloc failed by checking
the virtual address of the result, which will never fail. This patch
fixes it to check the actual result for failure.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>