OSV-2018-56 UNKNOWN READ in OT::propagate_attachment_offsets

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11013 Crash type: UNKNOWN READ Crash state: OT::propagateattachmentoffsets OT::propagateattachmentoffsets OT::GPOS::positionfinishoffsets...