Better handling of invalid font tables

Better handling of invalid font tables

Specifically when reading files with broken cmap tables, we could
get some undeterministic results. We handle this more gracefully
by verifying that the offsets are sane and bailing out early if not.
This replaces the current pattern throughout the font engine for
consistency.

This is a back-port of 4a1e5dbade4bab55f39bd368480dcca9a11e4b38
from Qt 5.

Change-Id: If4172b9ef0808801c8e27ffaad962535afe572ed
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Gbp-Pq: Name Better-handling-of-invalid-font-tables.patch