MFH: r468386

Import upstream commit fixing a crash caused by a null pointer
dereference.

Approved by: kde (tcberner)
Obtained from: https://github.com/annulen/webkit/commit/0e75f3272d149bc64899c161f150eb341a2417af
Differential Revision: https://reviews.freebsd.org/D15210

Approved by: ports-secteam (riggs)