When the same table is used for both base point and public point (which
is not a valid thing to do) then a corner case occurs when the table
point can be added to the same point. This has to be a double operation
instead.
The table point isn't able to be doubled as it has a z-ordinate of 0 and
the original point is overwritten with the invalid add result.
Fix this case by:
- copying the table point into the result,
- setting z-ordinate to Montgomery form of 1,
- double the result point in place.
c69665b999