(code_start): Initialize it to scanner_cursor,

not loc->end, since loc->end might contain garbage and this leads
to undefined behavior on some platforms.
(id_loc, token_start): Use (IF_LINTed) initial values that do not
depend on *loc, so that the reader doesn't give the the false
impression that *loc is initialized.
(<INITIAL>"%%"): Do not bother setting code_start, since its value
does not survive the return.