AWI commit of patch from Mark de Wever <firstname.lastname@example.org>
to check plsc->label_func for NULL where appropriate. I also
added a one-line comment change.
Tested by: Alan W. Irwin <email@example.com>. This was a
light test demonstrating the PLplot library and example 19 continued
to build without issues, and by demonstrating that running example 19
with the ps device under valgrind provided valgrind-clean results.