regression 1010: fix Clang warning and prevent unwanted optimization

Fixes the following Clang warning:

 os_test.c:906:3: warning: indirection of non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
                 *((uint32_t *) 0) = 0;
                 ^~~~~~~~~~~~~~~~~
 os_test.c:906:3: note: consider using __builtin_trap() or qualifying pointer with 'volatile'

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/ta/os_test/os_test.c b/ta/os_test/os_test.c
index 52d7b49..29c6814 100644
--- a/ta/os_test/os_test.c
+++ b/ta/os_test/os_test.c
@@ -903,7 +903,7 @@
 
 	switch (params[0].value.a) {
 	case 1:
-		*((uint32_t *) 0) = 0;
+		*((volatile uint32_t *)0) = 0;
 		break;
 	case 2:
 		*((uint32_t *)(stack_addr + 0x40000000)) = 0;