libmbedtls: mbedtls_mpi_exp_mod() initialize W
libmbedtls: mbedtls_mpi_exp_mod() initialize W

ae499f6a39f3 ("libmbedtls: mbedtls_mpi_exp_mod() initialize W") from
branch import/mbedtls-2.16.0

Initialize W using mbedtls_mpi_init_mempool() instead of memset() to
ensure that the temporary variable uses the designated memory pool if
configured.

Acked-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>
Signed-off-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os