mbedcrypto_missing: Avoid potential memory leaks as reported by csbuild

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/mbedcrypto_missing.c

@@ -150,6 +150,7 @@ int ssh_mbedcry_rand_range(bignum dest, bignum max)
     }
     rc = bignum_rand(rnd, bits);
     if (rc != 1) {
+        bignum_safe_free(rnd);
         return rc;
     }
     mbedtls_mpi_mod_mpi(dest, rnd, max);