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/lib/utils/mem_region.c b/lib/utils/mem_region.c
index 08bccf6..6bd78ba 100644
--- a/lib/utils/mem_region.c
+++ b/lib/utils/mem_region.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -32,7 +32,7 @@
 {
 	size_t i;
 
-	assert(tbl);
+	assert(tbl != NULL);
 	assert(nregions > 0);
 
 	for (i = 0; i < nregions; i++) {
@@ -114,7 +114,7 @@
 	uintptr_t region_start, region_end, start, end;
 	size_t i;
 
-	assert(tbl);
+	assert(tbl != NULL);
 	assert(nbytes > 0);
 	assert(!check_uptr_overflow(addr, nbytes-1));