Fix incorrect order of operations.

This results in an out-of-bounds access (if control_machine was not
being intentionally over-alloced to avoid it), the wrong address,
and other subtle problems.

C's order of operations meant this was resolving as:
i = (_backup_index() != -1);
which is either 0 or 1.

Through sheer luck, this still results in the correct answer for the primary
(_backup_index() is -1, and then i = (-1 != -1) is still 0 which is correct),
and first backup controller (_backup_index() is 1, and then i = (1 != -1) is
still 1 which is also correct), but any further backups controllers will end
up with the address of the first backup.