toke.c: Set PL_lex_state less when scanning formats

toke.c: Set PL_lex_state less when scanning formats

This (in yylex) is the only code that calls scan_formline:

    case LEX_FORMLINE:
	PL_lex_state = PL_parser->form_lex_state;
	s = scan_formline(PL_bufptr);
	if (!PL_lex_formbrack)
	{
	    formbrack = 1;
	    goto rightbracket;
	}
	PL_bufptr = s;
	return yylex();
    }

It is only reached when PL_lex_state is LEX_FORMLINE.  scan_formline
itself does not even look at PL_lex_state.  It does set it, though,
unless it has reached the end of the format (setting PL_lex_formbrack
to 0) or the end of input.

This means we end up flipping it back and forth between two values.

We don’t have to set PL_lex_state before scan_formline() at all.  Hav-
ing scan_formline only set it when it does not need to be LEX_FORMLINE
simplifies things, resulting in less code and fewer assignments.