Fix a hang that occurs when scanning a corrupt TrueType file.

Stop accepting -1 (EOF) as an array index when reading names from a TrueType file.

This relates in part to #1798.

> I think that a value of 0xFFFF would also be bad since it so frequently represents a non-existent or invalid value.

I see. I am probably nit-picking here, am ok with merging as-is.