[gh18096] assume worst-case for GOSUBs we don't analyse

[gh18096] assume worst-case for GOSUBs we don't analyse

During study_chunk, under various conditions we avoid recursing into
a GOSUB. But we must avoid giving the enclosing scope the idea that
this GOSUB would match only an empty string, since that could trigger
wrong optimizations (eg CURLYX => CURLYM in the ticket).

So we mark the construct as infinite, as in the code branch where we
_do_ recurse into it.