Added submodule contents into tree

externals/mbedtls/tests/configs/user-config-malloc-0-null.h

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+/* mbedtls_config.h modifier that forces calloc(0) to return NULL.
+ * Used for testing.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#include <stdlib.h>
+
+#ifndef MBEDTLS_PLATFORM_STD_CALLOC
+static inline void *custom_calloc(size_t nmemb, size_t size)
+{
+    if (nmemb == 0 || size == 0) {
+        return NULL;
+    }
+    return calloc(nmemb, size);
+}
+
+#define MBEDTLS_PLATFORM_MEMORY
+#define MBEDTLS_PLATFORM_STD_CALLOC custom_calloc
+#endif