AWI commit of patch from Mark de Wever <m.de.wever@datawell.nl> to check plsc->label_func for NULL where appropriate.  I also added a one-line comment change.
AWI commit of patch from Mark de Wever <m.de.wever@datawell.nl>
to check plsc->label_func for NULL where appropriate.  I also
added a one-line comment change.

Tested by: Alan W. Irwin <airwin@users.sourceforge.net>.  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.


svn path=/trunk/; revision=12302

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot