coverity: Fix MISRA null pointer violations

Fix code that violates the MISRA rule:
MISRA C-2012 Rule 11.9: Literal "0" shall not be used as
null pointer constant.

The fix explicitly checks whether a pointer is NULL.

Change-Id: Ibc318dc0f464982be9a34783f24ccd1d44800551
Signed-off-by: Zelalem <zelalem.aweke@arm.com>
diff --git a/bl1/aarch64/bl1_context_mgmt.c b/bl1/aarch64/bl1_context_mgmt.c
index 8be8830..210c358 100644
--- a/bl1/aarch64/bl1_context_mgmt.c
+++ b/bl1/aarch64/bl1_context_mgmt.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -69,7 +69,7 @@
 	security_state = GET_SECURITY_STATE(next_bl_ep->h.attr);
 
 	/* Setup the Secure/Non-Secure context if not done already. */
-	if (!cm_get_context(security_state))
+	if (cm_get_context(security_state) == NULL)
 		cm_set_context(&bl1_cpu_context[security_state], security_state);
 
 	/* Prepare the SPSR for the next BL image. */