don't recycle scanf format string if all conversions are done (bug #45808) * oct-stream.cc, oct-stream.h (scanf_format_elt::special_conversion): New enum value, no_conversion. (scanf_format_list::next): If not cycling through the list, return dummy scanf_format_elt after list has been exhausted. (octave_base_stream::do_scanf): Only cycle through the format list more than once if there are conversions to make and the limit on the number of values to convert has not been reached.