Add a bounds check to cairo_cff_parse_charstring()

The code in cairo-cff-subset.c parses a binary font format without seeming to bother much verifying the data. The result is that poppler can be used to cause an out-of-bounds access in cairo_cff_parse_charstring() 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: #444 (closed) Signed-off-by: Uli Schlachter psychon@znc.in