pp_match(): clarify intuit parens behaviour

pp_match(): clarify intuit parens behaviour

There was some dodgy code, (flagged by dmq), that as well
as setting RX_LASTPAREN(rx),  RX_LASTCLOSEPAREN(rx) to zero,
also set RX_NPARENS(rx) to zero.

The actual logic is that if we reach that point, the pattern shouldn't
have any capturing parentheses, so instead of assigning zero to it, assert
that RX_NPARENS(rx) is zero.