Wipe stack buffers in block_cipher_df

This is a partial backport of
1b36499062 (only for the buffer wiping).
Other wiping calls were previously added as backports of
"CTR_DRBG: clean stack
buffers" (d9aa84dc0d). This completes
the backporting of stack buffer wiping from the development branch.

library/ctr_drbg.c

@@ -226,6 +226,10 @@ static int block_cipher_df( unsigned char *output,
 
     mbedtls_aes_free( &aes_ctx );
 
+    mbedtls_zeroize( buf, sizeof( buf ) );
+    mbedtls_zeroize( tmp, sizeof( tmp ) );
+    mbedtls_zeroize( key, sizeof( key ) );
+    mbedtls_zeroize( chain, sizeof( chain ) );
     return( 0 );
 }