Provide and use internal function mbedtls_zeroize_and_free()

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/platform_util.c b/library/platform_util.c
index c67b80d..4a9a749 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -129,6 +129,15 @@
 }
 #endif /* MBEDTLS_PLATFORM_ZEROIZE_ALT */
 
+void mbedtls_zeroize_and_free(void *buf, size_t len)
+{
+    if (buf != NULL) {
+        mbedtls_platform_zeroize(buf, len);
+    }
+
+    mbedtls_free(buf);
+}
+
 #if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
 #include <time.h>
 #if !defined(_WIN32) && (defined(unix) || \