Fix pointer aliasing in bignum tests

Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index c67d653..b06aec9 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -993,9 +993,7 @@
  *                 to an initialized MPI. This value can be negative, in which
  *                 case a positive answer will still be returned in \p X.
  * \param N        The base of the modular inversion. This must point to an
- *                 initialized MPI and be greater than one. If this points to
- *                 the same MPI as \p X, then the value returned in \p X will
- *                 be incorrect.
+ *                 initialized MPI and be greater than one.
  *
  * \return         \c 0 if successful.
  * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.