Implement and test mbedtls_mpi_mod_random

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 0057eba..3eef4e7 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -214,6 +214,17 @@
 
 /* BEGIN MERGE SLOT 6 */
 
+int mbedtls_mpi_mod_random( mbedtls_mpi_mod_residue *X,
+                            mbedtls_mpi_uint min,
+                            const mbedtls_mpi_mod_modulus *N,
+                            int (*f_rng)(void *, unsigned char *, size_t),
+                            void *p_rng )
+{
+    if( X->limbs != N->limbs )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+    return( mbedtls_mpi_mod_raw_random( X->p, min, N, f_rng, p_rng ) );
+}
+
 /* END MERGE SLOT 6 */
 
 /* BEGIN MERGE SLOT 7 */