Fix tracing output of PARSE (cc#2150)

This is a fix to properly zero-terminate strings in the internal `Trace_String` helper used for e.g. tracing output in PARSE.

This fixes [CureCode issue 2150](http://issue.cc/r3/2150).

Fix written by Shixin Zeng: zsx/r3@25d35a946269c6547b1f1091b10753ccc4b91216