arch/x86: Handle smm_subregion() failure

The callers don't necessarily check return value of
function. Make sure the parameters are not left
uninitialised in that case.

Change-Id: Ic02db2d35b2ec88506320e7df609940de4aef005
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34708
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/icelake/memmap.c

@@ -70,6 +70,8 @@ int smm_subregion(int sub, uintptr_t *start, size_t *size)
 		sub_size = ied_size;
 		break;
 	default:
+		*start = 0;
+		*size = 0;
 		return -1;
 	}