Fix hang when using errorbar with empty dataset (bug #45554). * __errplot__.m: Use concatenation to append new errorbar plots to retval, rather than assignment which can fail if the RHS is empty. * __errplot__(__do_errplot__): Determine number of plots by checking number of columns in dataset, but only if dataset is not empty (row is zero for example). Only process a legend key if the number of plots is greater than 0.