Fix incorrect count of netwm atoms

This fixes a regression introduced with
59cb063b99e6ec13c0bc3cf3fd95a01258184e86.

The commit added 4 new atoms but only incremented the atom count by 3.
Which can cause a crash as soon as the NET classes are used.

To ensure that such an error is not introduced again, an assert is
added.

BUG: 339771
REVIEW: 120539