cmGrPlot.c : Fixed apparent but in _cmGrPlotObjIsSelected()

Changed: memset(a,0,sizeof(a))
to:      memset(a,0,sizeof(*a));

cmGrPlot.c

   cmGrPlObjH_t    oH;
   oH.h = op;
 
-  memset(a,0,sizeof(a));
+  memset(a,0,sizeof(*a));
   a->ctx   = op->p->ctx;
   a->cbArg = op->cbArg;
   a->selId = selId;