www/firefox et al.: backport Wikipedia crashfix for clang 3.5

[jbeich]

Summary

Backport Wikipedia crashfix for clang 3.5

PR: 196802
Reported by: dumbbell
Obtained from: https://github.com/mozilla/gecko-dev/commit/87f367b
Obtained from: https://github.com/mozilla/gecko-dev/commit/c97894b (esr31 version)
Approved by: flo (mentor)
MFH: 2015Q1

Test Plan

No releases besides rolling 11.0-CURRENT have clang 3.5. So,

1/ poudriere jail -s
2/ allow X11 apps
3/ install firefox
4/ firefox -no-remote en.wikipedia.org

[flo]

Approved

[jbeich]

Closed by commit rP377564 (authored by @jbeich).