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.

In you want to check for overflow, I think it should be done while bitshifting [here](https://github.com/fontforge/fontforge/blob/1d9d28c3c3b4eca0a62f1ea352d866da9e91ddd9/contrib/fonttools/showttf.c#L158). But is overflow checking really needed?