cmocka: examples: Fix assert macros usage

Replace used macros with more suitable
for given context.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
diff --git a/example/calculator_test.c b/example/calculator_test.c
index a2dd21d..ab8cad8 100644
--- a/example/calculator_test.c
+++ b/example/calculator_test.c
@@ -159,10 +159,7 @@
 static void test_find_operator_function_by_string_valid_null_functions(void **state) {
         (void) state; /* unused */
 
-  assert_int_equal(
-          cast_ptr_to_largest_integral_type(
-              find_operator_function_by_string(0, NULL, "test")),
-          0);
+	assert_null(find_operator_function_by_string(0, NULL, "test"));
 }
 
 /* Ensure find_operator_function_by_string() returns NULL when searching for
@@ -176,12 +173,8 @@
 
         (void) state; /* unused */
 
-    assert_int_equal(
-            cast_ptr_to_largest_integral_type(
-                find_operator_function_by_string(array_length(operator_functions),
-                                                 operator_functions,
-                                                 "test")),
-            0);
+	assert_null(find_operator_function_by_string(
+	        array_length(operator_functions), operator_functions, "test"));
 }
 
 /* Ensure find_operator_function_by_string() returns the correct function when