[PATCH] ec: fix left shift overflows on WIN64 build

[Niibe Yutaka]

Jussi Kivilinna via Gcrypt-devel wrote:
> * mpi/ec.c (ec_mulm_448): Cast constants to (mpi_limb_t) before
> shifting left by 32.

Thanks.  When I wrote that, I forgot to consider about LLP64 system.
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