The read_prefix_1 function has several return statements springled
throughout the code. As we have to free memory upon getting an error,
the free code has to be repeated at every single retrun -- which it is
not, so we have a memory leak here.

Refactor the code to use the typical `goto out` pattern, which will free
data when an error has occurred. While we're at it, we can also improve
the error message thrown when multiple ambiguous prefixes are found. It
will now include the colliding prefixes.