[truetype] Fix handling of packed deltas in Variation Fonts.

* src/truetype/ttgxvar (ft_var_readpackeddeltas): Don't expect the number of
bytes used to encode the deltas to be higher than the number of encoded
values.  The specification allows a very compact encoding; for example, a
list of 200 zeros can be encoded with just a couple of bytes.

We now count the consumed bytes to make sure to not read more than expected.
1 file changed