Use mbedtls_platform_memset in data_randomize

More secure memset should be used here instead
of standard memset.
diff --git a/library/aes.c b/library/aes.c
index 4230b25..c96f29e 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -541,7 +541,7 @@
     int num;
 #endif
 
-    memset( tbl, 0, tbl_len );
+    mbedtls_platform_memset( tbl, 0, tbl_len );
 
 #if AES_SCA_CM_ROUNDS != 0
     // Randomize SCA CM positions to tbl