Make ecp_select_comb() create valid EC point with Z coordinate set
Make ecp_select_comb() create valid EC point with Z coordinate set

ecp_select_comb() did previously not set the Z coordinate of the target point.
Instead, callers would either set it explicitly or leave it uninitialized,
relying on the (only partly upheld) convention that sometimes an uninitialized
Z value represents 1.

This commit modifies ecp_select_comb() to always set the Z coordinate to 1.
This comes at the cost of memory for a single coordinate, which seems worth
it for the increased robustness.

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>

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