[auxiliary] robustify font property lookups

This adresses an issue with entries in the font table that lack the
common font structure (manually parsed fonts??). Subtable lookups
are now guarded with nil-checks.

Reported by Herbert Voss

Signed-off-by: Philipp Gesang <phg42.2a@gmail.com>
[fontloader] sync with Context as of 2014-09-26
Merge pull request #254 from phi-gamma/2.5-fixes

2.5 fixes