The code in cairo-cff-subset.c parses a binary format without seeming to
bother much with verifying the data. The result is that poppler can be
used to cause an out-of-bounds write in cairo_cff_font_read_fdselect()
via a crafted font file. Fix this by adding the needed length check.

The other code in the file also contains lots of similar things. Since I
cannot really fix everything properly, I'll just fix the one instance
that was found by a fuzzer.

No testcase is added, because this depends on a broken font that is
quite large. Adding something this big to the test suite does not seem
sensible.

Fixes: cairo/cairo#451


Signed-off-by: Uli Schlachter <psychon@znc.in>