Add the error handling if the effective zoom is close to zero.
The zoom factor is stored as a fixed point number, and the effective zoom may
be close to 0 (because we have to mulitply the zoom values of ancestors).
So we have to make sure we don't divide by zero because the effective zoom
could be zero.
Differential Revision: https://phabricator.services.mozilla.com/D224551
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1922339
gecko-commit: 7ad44be1dfac1c831a3ed62658bb6dfc2851a70b
gecko-reviewers: layout-reviewers, firefox-style-system-reviewers, emilio