Prevent empty segments from being added for syllable breaks

[rdavidson1994]

The -p argument to input phones is meant to allow using '-' to indicate syllable breaks. Right now, this always fails with this error message:

> .\flite -p "pau ih0 - n k r ax0 - m eh1 n t - ax0 l pau"
VAL: tried to access string in -1 typed val

The problem is that tokentosegs unconditionally adds a new segment to the utterance for every token. In the case of '-', this segment never has its name set and is never added to the syllable structure, which breaks some assumptions later on. Moving the relation_append call past the check for '-' solves the problem.