bliss: Make sure sampler exists after checking for it earlier

src/libstrongswan/plugins/bliss/bliss_private_key.c

@@ -384,7 +384,7 @@ static bool sign_bliss_with_sha512(private_bliss_private_key_t *this,
 
 end:
 	/* cleanup */
-	sampler->destroy(sampler);
+	DESTROY_IF(sampler);
 	hasher->destroy(hasher);
 	sig->destroy(sig);
 	fft->destroy(fft);