ensure qw//, q'' and '' report line on a missing terminator

ensure qw//, q'' and '' report line on a missing terminator

scan_str() only sets PL_multi_end on success, but the qw, q amd '' cases
were calling the COPLINE_SET_FROM_MULTI_END; macro before reporting
failure, overwriting the line used for reporting errors.

For the simplest case, as in the ticket and the tests, PL_multi_end is
zero, so the error is reported without a line number.