ec-nist: avoid unintentional conditional branch by comparison

* mpi/ec-nist.c (_gcry_mpi_ec_nist521_mod): Use mpih_limb_is_not_zero.
* mpi/mpi-internal.h (mpih_limb_is_not_zero): New.

Cherry pick master commit of:
a9e7aa647e4b84964c76230370d71235383e5c2d

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>