Provide and use internal function mbedtls_zeroize_and_free()

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index bf72c18..eff5627 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -253,8 +253,7 @@
     mbedtls_mpi_core_to_mont_rep(X, X, N->p, N->limbs,
                                  N->rep.mont.mm, N->rep.mont.rr, T);
 
-    mbedtls_platform_zeroize(T, t_limbs * ciL);
-    mbedtls_free(T);
+    mbedtls_zeroize_and_free(T, t_limbs * ciL);
     return 0;
 }
 
@@ -270,8 +269,7 @@
 
     mbedtls_mpi_core_from_mont_rep(X, X, N->p, N->limbs, N->rep.mont.mm, T);
 
-    mbedtls_platform_zeroize(T, t_limbs * ciL);
-    mbedtls_free(T);
+    mbedtls_zeroize_and_free(T, t_limbs * ciL);
     return 0;
 }