update comments and add tests for cJSON_SetValuestring

cJSON.c

@@ -397,6 +397,7 @@ CJSON_PUBLIC(double) cJSON_SetNumberHelper(cJSON *object, double number)
     return object->valuedouble = number;
 }
 
+/* Note: when passing a NULL valuestring, cJSON_SetValuestring treats this as an error and return NULL */
 CJSON_PUBLIC(char*) cJSON_SetValuestring(cJSON *object, const char *valuestring)
 {
     char *copy = NULL;
@@ -405,7 +406,7 @@ CJSON_PUBLIC(char*) cJSON_SetValuestring(cJSON *object, const char *valuestring)
     {
         return NULL;
     }
-    /* return NULL if the object is corrupted */
+    /* return NULL if the object is corrupted or valuestring is NULL */
     if (object->valuestring == NULL || valuestring == NULL)
     {
         return NULL;

tests/misc_tests.c

@@ -444,6 +444,7 @@ static void cjson_functions_should_not_crash_with_null_pointers(void)
     TEST_ASSERT_FALSE(cJSON_Compare(NULL, item, false));
     TEST_ASSERT_NULL(cJSON_SetValuestring(NULL, "test"));
     TEST_ASSERT_NULL(cJSON_SetValuestring(corruptedString, "test"));
+    TEST_ASSERT_NULL(cJSON_SetValuestring(item, NULL));
     cJSON_Minify(NULL);
     /* skipped because it is only used via a macro that checks for NULL */
     /* cJSON_SetNumberHelper(NULL, 0); */